I think most early users do check further than open source licenses. It’s possible they’ll add things later, but if they add after it has enough users we have significant number of users to have some people check. And if the user base is small then they’re probably more involved, or are reading/modifying code for their use cases.

Of course it’s not foolproof, but it has worked for a long time because of things like that

I think for a open source projects with such a low number of users, the first few users will definitely look further than “it’s open source”.

That’s the thing though if it’s open source and 99.9% don’t check that 0.1% checking it will be enough.