It mean be polite. If I tell you that libfoo is super secure, accept that as true, continue to listen to the sales pitch, and then in the evening, look at the source for it so you can assert for yourself that it is secure.

Don't derail the whole thing by arguing that libfoo couldn't possibly be secure because x, y, and z, it's about getting to the vendors value proposition and not getting stuck in important but not-right-now details.

Never trust (software/code/people) until you have investigated it yourself.