Do I understand correctly that reface YC completely ignores key velocity, regardless if it's being controlled by the internal keyboard or through midi?

That is correct. It is an organ.

Does it send velocity when used as midi controller?

Yest. It sends velocity, 1-127, when used as a MIDI Controller

Do I understand correctly that reface YC completely ignores key velocity

Not exactly, if press a key softly will sound a main organ tone, but no percussion attack, even if the that swith is ON position. Just how my old vintage organ from 70s. This is fun feature!