Its more than just that, they have to understand it inside out enough so they can pick the way of explaining it that suits the target audience.,...
..I remember my sadly departed Cambridge PhD physics master, and I...he was explaining relativity and how there was no absolute frame of reference for motion - all was relative.
I had a thought. 'But what about rotation. A skater's arms and legs fly out when she spins, but what is she spinning relative to?'
There was a significant pause whilst he thought up the best reply
'The average fixed mass of the rest of the universe' he said.
WOW!
THAT is not only lknowing your subeckt but knowing your boy as well.
.Too many 'experts' are third rate minds who haven't actually got to the stage of understanding everyone else, and then thinking for themselves.
To many 'experts' are simply parroting received wisdom.
Take Brian Cox as an example. Unlike my physics master he doesn't say 'this is how science makes a model of it, that seems to work'. No. He says 'this is how it is, trust me, I'm a physicist'
F*ck off, Brian.