You make a great point, and the title of the article is misleading at best, really just plain wrong IMHO. But the title also doesn’t state he isn’t invested in the stock market, only that he doesn’t own any individual company stock.
Personally, I think this version is the most fair way for politicians. On one hand, any investment for them gives them incentive to put market health (money) over their constituents. On the other hand, investing in a 401k or mutual fund is something everyone should do, how else will you afford retirement?
Frankly I feel like the better option is to just not let people borrow based on stocks at all. Even if you paid in at X price, there’s no guarantee it’ll still be at X price or greater when the loan comes due, so to speak.