There's no report from any investors who suggest they were confused about IP/original-work in any way. There is an assertion to the contrary in the article: "Stability told Sifted that the investors that backed the company were fully aware of Stable Diffusion’s IP ownership."

So while it's possible investors just don't want to admit they didn't look too deep, it's also possible everyone relevant knew exactly what was being offered: a fast-moving team of opportunists, adapting core tech originating elsewhere. That alone is plenty valuable in a hot area. 'Fast-followers', rollups, & integrators can do great in a tech boom.

So, this story might mainly be resentful competitors/employees taking potshots. (Or maybe even: potential next-round investors trying to win for themselves a lower-valuation via media spin.)