Plywood is no longer advised or recognised for British Standards..I understand and totally see where you are coming from, it's only after he fitted the plywood where I was doubting whether it was correct as it was quite flimsy, however as he's screwed it down every 150mm it seems pretty solid. But having said that from what I've been told here and researching its wrong
S1 adhesive is all about the deformability of the adhesive.. that will flex all day long, but a tile doesn’t bend or flex at all.. if the floor has movement then the tiles will tent or pop
if the prep isn’t done properly then you have problems down the line..
the concrete would need priming but as long as clean and solid and dry it’s fine to tile onto