Big tech interviews aren't team specific or even experience specific (with some exceptions). Teams do a wide variety of work, including stuff similar to low latency C++/Java, and team selection doesn't happen until after passing interviews.

It's not really a question of relevant experience at all - it's about passing the standardized exam that is a big tech interview. Experience comes more into play when doing team selection afterwards.

Was on a team this happened on and a lot of the l4 l5 Eng gossiped about how little our L7 knew

I don't think someone gossiping about how their work superiors are fools is really notably strange to be honest.

I dunno where you worked but this is def not normal at least within technical ladder ime