I'm not really sure which posts you refer to. Job posting tend to mention large frameworks (such as django or rails) or ask for experience with certain vendor

I'm not really sure which posts you refer to. Job posting tend to mention large frameworks (such as django or rails) or ask for experience with certain vendor software (such as postgres or redis). Specific libraries would be highly uncommon, as they would only be used for a single task (and not what you deal with every day). I'd be wary of any job posting that does this: they could just have a single problem that they'll get you to solve as an interview question.