I’m not sure what level of coverage is normal, but it’s not unheard of to ask for a written opinion from a legal firm saying “this business model is legally sound”.
In this case it sounds like this was not asked for, and they just took the founders’ word for it.
On the other hand worth noting that “taking their word for it” happens to some degree in almost business deals; after all, past a certain point, outright lies will probably land you in jail (or at least with a massive fine).
It could be a sloppy DD process was run here, or it could be that this sort of thing happens infrequently enough that it’s not worth applying a fine-toothed comb to every single claim.