This isn't true. Best I can guess you are referencing the company formerly-known-as-twitter. The 90% number refers to software engineers, not employees and only applies to one of several companies that Musk runs.
EDIT: in general, my point is that he practically triggered mass layoffs at large companies by being so vocal about doing it at twitter. A ton of investors were celebrating it after and pushing companies to do the same.
The market for software engineers crashed after https://fred.stlouisfed.org/series/IHLIDXUSTPSOFTDEVE
Also see the layoff charts tab on this site https://layoffs.fyi/
His acquisition was in October 2022. Job postings started dropping off in May 2022. It was already leveling off as early as February 2022. You'd have to squint really hard to believe this was caused by musk, or that he played a major factor.
That's an interesting claim, and one I'd be curious to see a more detailed argument for. I'm sure it had an impact, but arguing it had a larger impact than the prevailing market conditions seems hard. The massive overhiring in the year or two before seems like the obvious culprits for the majority of the mass layoffs.
I see a lot of hyperbolic and flat out false things said about Musk. I push back on them because I think that people's habitual innaccuracy when he is involved tends to make the real criticism of him harder.