That's all very well, but often people haven't thought of whatever you might tell them. By saving them from possible offence[1] you're limiting what ideas they have available to use. When I was running a startup I was more than happy to hear the same advice and ideas over and over again from mentors because just occasionally you got something that was new that you'd not considered before. That was far better than being saved from listening to people who might be suggesting things I'd already considered.

Advice and ideas that came from proven and experienced people were often better and more useful but only because that meant I didn't have to spend so much time researching them to see if they might work for me or figuring out the potential pitfalls - the ideas themselves were no better or worse than if they'd come from someone "unqualified".

Regarding the 'impostor syndrome' problem, that doesn't really enter into the equation if the person listening to your ideas and advice is sensible. No one just blindly goes and does what someone outside of their startup tells them to do, regardless of how much respect they might have for that individual.

[1] Anyone who gets offended by someone offering well-meaning advice is an idiot and probably wouldn't have listened even if that advice was based on a mountain of prior experience.