Reach is measured to the top of the headtube isn’t it? Am I right to assume that of two bikes bikes with identical reach numbers, the one with the shorter headtube is actually shorter in front centre length in reality?

Would it not be longer? All other things being equal?

The longer the headtube the further back it effectively puts the bars?

BB needs to be moved forwards to keep reach the same for a shorter headtube, and vice-versa. Not by much, though.

Reach is measured to the top of the headtube isn’t it?

Yes, top dead centre.

Am I right to assume that of two bikes bikes with identical reach numbers, the one with the shorter headtube is actually shorter in front centre length in reality?

For the first second time ever, you are indeed right! 😉 And if both have equal BB drop and fork lengths, then the bike with the shorter head tube will also be shorter in reach with the bars at the same height.