In the OP example, it seems the senior dev is saying “on my authority”. And sometimes that is enough, especially if the senior dev can give examples of when not following this practice bit them.
But sometimes there is a higher authority, such as “it’s what is recommended in Google’s SRE book”, which is probably good advice if you are building a large SRE team. (Though as you say, not in all situations.)
I think in the worst case, “best practice” can indeed be used to shut down discussions of a leader’s preferences. But you can smell that out by asking for concrete examples, and asking how widely the practice is recommended.
A good “best practice” should be justifiable and explainable.
All that said, sometimes as the senior engineer you need to go with gut feel; “this design smells like it will give us trouble within a year” is the sort of thing I sometimes say. But I think it’s important to be honest that it’s a hunch, not a certainty in these cases, and discuss/weight accordingly.