That strikes me as an exaggeration, or at least misleading(*). For example, a quick check shows that something like 1/4 of OTC stocks are above $1, while about 3/4 are penny stocks. Sure some of the higher ones will go out of business, but a few of the penny stocks will turn into real businesses too and end up in to top group. I do agree that bankruptcy isn't what to focus on - stocks can easily become nearly worthless without BK, and likewise, BK stocks don't always end up worthless and may do some restructuring such that the old equity is still worth something in the post-BK company.
* - would it still be a true statement if you said 90% of all stocks end up virtually worthless? There's a lot of survivorship bias over a long enough time period. For example, it is estimated that 90% of all people that ever lived have died, while the present population is 10% of all humans that ever lived.