There's a lot of spin going on in all camps, including the Sanders camp. This is par for the course in a political election year.
The article you cited is guilty of the same criticisms it makes of other articles. It fails to provide any concise information about how many voters in the 18-29 age range in 2020 turned out compared to 2016.
In fairness, this is partly because election officials do not track voters by age when they vote. So the only way to know anything about this kind of metric is to look at the same exit polls the article rejects as inaccurate.
The Sanders campaign, like other campaigns, do their own polling and sampling. So they have pretty much the best idea of anyone as to how well they've really done. So listen to what Bernie himself said:
"Have we been as successful as I would hope in bringing in young people in? And the answer is 'no'," he told reporters at a press briefing on Wednesday.
"We're making some progress but historically everybody knows that young people do not vote in the kind of numbers that older people vote," Sanders went on. "I think that will change in the general election. But to be honest with you, we have not done as well in bringing young people into the process. It is not easy."
However, that's not the only thing we can infer from available data. If you put together a spreadsheet state-by-state to tally vote totals compared to votes received by Sanders, you will see a strong pattern. The data shows that Sanders' overall vote count is running 20-30% LOWER than in 2016 in most states that have voted so far, despite the fact that across-the-board vote count is running 10-30% HIGHER than in 2016. Sanders defenders argue that this is because the field was so much bigger and stronger than in 2016, but I see this as a throwaway argument. If Sanders was really a strong vote-getter, he wouldn't be bleeding support compared to 2016 because the ones who voted for him then already knew him, whereas his opponents this year were mostly newcomers to the primaries. He already had name recognition that the others didn't have among Democratic voters.
So what does this broader insight tell us? Well, all of the polls we've seen say that Sanders' support levels are strongest among the 18-29 age range. So if he's hemorrhaging that much voter support in the overall numbers, it either means that the number of voters in the 18-29 range matched 2016 while the rest of his supporters disappeared almost completely into the woodwork, or (much more likely) his support among 18-29 year olds sagged compared to four years ago along with the other age groups.
Given Sanders' remarks Business Insider quoted, I'd go with the latter likelihood.