I believe it is the new interpretation as you mentioned.

I think he was trying to head it and his hand was in a somewhat natural position. The contact between head/hand almost looked simultaneous to me. Either way, had the ball gone in from that, it would've been reversed. (Well, I say that, but you never know with this horseshit VAR process in the PL.) Because he didn't score and it didn't appear to be intentional, play was allowed to continue and the resulting goal stood.

To answer your question, I think it tends to be very obvious when it is intentional. Salah's motions looked natural to me, but I admit to being biased.