If someone asks you 'why?' can you tell them? If not, you are using it as a weasel word. If so, then you are using it as shorthand for 'I could explain it but I don't think I need to right now'.
Or literally that’s the only information what you really have. I haven’t met a single person except universities who knew more about this topic (ie they’re rare). Even the library/interface which is used is not well understood usually, even on the surface level.
It's not clear, even, to what depth this 'best practice' applies. Writing your own crypto primitives is probably a bad idea, but what about combining them? AEAD approaches demonstrate there can be nuance even with battle-tested primitives and how they're combined or used in practice. Oh, but what about key derivation or protecting the keys in general? What good is that library's encrypt method if the DIY key secrecy/rotation/exchange is sloppy?