Zelensky claims that North Korea plans to send 100K troops. This may or may not be real. Reportedly from your wiki source there are only "300K+" Russian combatants in Ukraine. So, 100K extra troops would be bigger deal, i.e. like a 25% force boost, assuming the North Koreans won't stick to the Kursk region, which they may or may not. Even if we take Putin's June 14, 2024 claim of 700,000 Russians in the combat area, 100K is still a 14% boost. (Somewhat tangential, but AFAICT, Ukraine's intelligence officials have generally said that Putin overstates the number of Russian soldiers in Ukraine.)
FTWT, one Ukrainian 'expert' claims that the threat of being hit with ATACMS in assembly areas might cause Kim Jong Un to reconsider sending more troops to the front. I'd say that needs to be taken with the appropriate large measure of salt.
The closest thing to the US calling it a big deal I found is this:
In a press briefing Monday, US State Department spokesperson Matthew Miller said the involvement of North Korean troops in the war was “a major escalation by Russia, bringing in an Asian military to a conflict inside Europe.”
Clearly he isn't talking about numbers, but mostly symbolism thus far.
Russia is understandably more muted on this. But they did say they wanted 500K North Korean workers.
As for the connection with ATACMS strikes in Russia. I've not been able find anything truly official, but some of the press said that some "officials" have made the link, off the record. BTW, ATACMS also happens to not be the only escalation by the US recently. They've also approved sending anti-personnel mines to Ukraine now. At least for the latter a more clear reason was officially provided.
Secretary of Defense Lloyd Austin [...] told reporters on Wednesday that the decision was motivated by Russia’s changing tactics on the frontlines.
“Because the Russians have been so unsuccessful in the way that they have been fighting, they’ve kind of changed their tactics a bit,” Austin told reporters in Laos.
“They don’t lead with their mechanized forces anymore. They lead with dismounted forces who are able to close and do things to kind of pave the way for mechanized forces,” he explained.
Austin said Ukraine’s military needs “things that can help slow down that effort.”
BTW, Russia now rather claims ATACMS are no big deal either because they say they shot down most of them. The US thus far seem not have confirmed ATACMS policy change on record (as opposed to off the record)
The Biden administration hasn't publicly confirmed the ATACMS policy change. State Department spokesperson Matthew Miller told journalists at a Monday briefing he would not confirm or deny approval for ATACMS use inside Russia, but said the U.S. response to Russian and North Korean military cooperation in the war "would be firm."
Likewise:
In a Monday morning briefing in Rio de Janeiro, U.S. Deputy National Security Adviser Jonathan Finer evaded questions from reporters about Biden's [ATACMS] authorization.
"I'm not confirming any decisions that have or have not been made about U.S. assistance when it comes to these operational questions," Finer said. "I will say, with regard to the comments that came out of Russia, the fire was lit by Russia's invasion of Ukraine."
And anyhow, Ukraine apparently received a rather limited supply--around 50 of them ATACMS.
I'll also note here that the US statement in May that coincided with allowing GMLRS strikes into Russia was pretty vague. Officially little was disclosed what Ukraine was allowed to do and what it still wasn't. Only by observing actions over time the limits were inferred by the press.
Also, this last bit is entirely my speculation, but with Trump slated to take office in a couple of months, Biden's team may have decided to call bluff on Putin's claim that he'd regard a strike with longer range US missiles by Ukraine as direct involvement of the US in the war. What's Putin going to do now? Hit some US targets? Formally declare war on the US? Neither of these actions are likely to sit well with Trump. There's a fair bit of expectation that Trump will lower US support for Ukraine. But if Russia were to escalate directly against the US now, all that might go out the window, even when/after Trump takes office. So, like with many other escalations before this one (Western tanks, HIMARS, HIMARS [meaning GMLRS] strikes into Russia etc.) we might see the usual pattern with Russia warning about WW3 before such a step is taken (and more or less vaguely threaten direct strikes on Western targets) and then downplaying the escalation by Western supply (or rules on use that come attached) after it happens.
Another angle is that reportedly the US relaxation of rules was also followed by the British. And since they've promised to stick with Ukraine even if Trump changes tack, that's also a way for Biden to push the boundaries beyond the end of his mandate. Once Storm Shadow strikes into Russia are 'normalized' now, it would be harder for anyone to call WW3 over them later. Anyhow, the British government likewise "refused to comment on the reports for operational reasons".