It depends on how you determine market cap on private equity. $10M still seems like a small business. $100M might, and could be consistent with the worker limit found in other small business definitions (<200 people).
If they are legitimate businesses, then I don't see the problem with having a portfolio of them. Each one can be a significant risk. If they aren't legitimate, then additional regulations should be formed. If you still think it's a problem to have a portfolio, then they could change the language to use the limit as an aggregate of all businesses owned/sold.
"I'm at the point of telling all the billionaires who somehow can't figure out what their assets are worth for a wealth tax"
It's not just them. Forbes has this issue when ranking them. The IRS needs to be able to validate the numbers too, which is unlikely.