If we take it to be the case that A knows that p entails p - and this is a generally accepted truth - then write what you know entails that no one can write confidently about FTL travel.
Yep. Indeed, FTL is hardly the only subject where that applies.
The question one needs to ask oneself is there, "If I cannot write confidently about this... am I sure I should address it?" Is the narrative really better for having the technical discussion in it? It is a pretty basic risk vs reward question.
And, I'm being broad in my application of "technical" - f'rex, sociology is technical, to a sociologist. And while on Trek they reverse he polarity of the neutron flow every other week, many great bits of sci-fi work to avoid the technobabble..
There are cases of deliberate departure from the principle. JRRT, in LotR, deliberately writes a history and sociology that he (given his academic training) would have known makes no sense. He had artistic reasons for doing that.
Every work has its rhetorical goals. You use the bits that serve those goals, and sacrifice those that don't on the alter of prose.