Interesting claim in the source you cited, although it's really more of a blurb than an article. They even call it a "fun fact", which is far from an investigative report or something. Also, the source seems to be in the business of "helping" small businesses that are new to importing freight containers which makes me question their motives to be honest.
Allow me to inject into this thread some information about the technical and administrative features of shipping intermodal freight containers internationally.
There are two main cases:
set up a sham business that pretends to be a legitimate importer, but actually puts contraband in the containers.
sneak your contraband into the shipment of another legitimate company who has a container enroute to the US.
For case 1, setting up a sham business
In this case, the safeguards against money laundering are the most likely finder of your misdeeds. Highly unlikely to succeed. The "fun fact" in your cited article even acknowledges this in this part:
In fact, it’s easier to list why your container may be tagged for inspection than >why it wouldn’t. Here are a few:
If you’re a virgin importer, US Customs may inspect your first few shipments to >establish credibility
.....
For case 2, sneaking your goods into some else's shipments
This is actually much more common, and so more interesting, at least to me.
Who has the opportunity to sneak some contraband into a container that is being shipped to the US from an unwitting company somewhere overseas? The company's shipping employees. The truck driver (or two, or three) who delivers the container to the point of debarkation. Someone following the truck driver who knows the container's destination, and tampers with the shipment at a rest stop. Every employee at the port while the container sits there waiting for its ship. Crew members on the ship -- maybe, depends on if the container is accessible, if it's buried at the bottom of a stack then crew members won't be able to help.
Ok your shipment has cleared customs, now you need to get it out of the container before your mule company gets wise. You going to pay off a port workers in the receiving country? A truck driver? How can you guarantee the driver on your payroll gets assigned the right container?
Assuming all your nefarious plans go perfectly, what stops this from happening?
In fact, it’s easier to list why your container may be tagged for inspection than >why it wouldn’t. Here are a few:
....
If the weight of your container doesn’t correspond to the packing list
Typical freight scales on a crane or container handler round to the nearest 50lb (23kg), so if your shipment is >26lb(12kg) more than the paperwork, that container will be scrutinized. You're going to have to pay off a lot more truck drivers or port workers if you can only put 20lb of contraband in each container.
Also, there's this, which is worse: containers have tamper evident seals with unique serial numbers. Interlopers cannot open and re-close containers without leaving visual evidence. The seal serial numbers are on the paperwork. Here is an example that anyone can buy:

Here's a video about seals and how to apply them
https://youtu.be/f4R2F7yU8AY
And here's a video about trying to defeat a seal, which I think is debunked by the first video. Clamping a seal in a drill chuck will surely cause exactly the deformation the first video warned you to check for.
https://youtu.be/Cv16Skf7PzE
There are a few other "attacks" on container seals that I can assure you customs inspectors are well trained on detecting since even a fool like me has been trained on them. And I don't work in customs.
And one more thing: the containers themselves have features to prevent you from opening or taking off the doors in lieu of breaking a uniquely serialized tamper evident seal.
- They have "customs catches"
The serialized seal is typically on the right side of the door. There is a piece of steel overlapping the left side that prevents the left side from opening first. So you cannot open the left side even though there is no seal on the left side. This piece of steel is either welded or attached with carriage bolts that can't be opened from the outside.
See the top right caption of this diagram:

What prevents you from disconnecting the vertical securement bars from the door panels? If you look at the hinge brackets, you will see that each one is held on with either 4 or 6 bolts. And of those 4 or 6, one of them will either be tack-welded to the door panel, or else will be a carriage bolt that cannot be removed from the outside. After doing this operation and replacing it, there will be obvious visual unmistakable evidence to any customs inspector that the container has been tampered with. Paint fades. Your new paint will stand out like a sore thumb. Unfortunately I can't find a good picture of this using Google images. Find a container and see for yourself though.
What prevents you from tapping the hinge pins out of the hinges and removing both doors as a single big piece? Those hinge pins are either tack-welded or otherwise secured. The same concerns as the last paragraph apply. Your paint touch up after inserting the contraband will be painfully obvious. Can't find a picture of this either, so go find a container.
And consider this: a used freight container is pretty cheap. Nobody touches up the paint on them because it's not worthwhile. By the time the paint needs retouching, the container is only worth a couple of thousand US dollars. Repaint costs much more. So evidence of paint retouching on a freight container is a sure ticket to a thorough customs inspection.