i thought you could nt put hand over mouth

It has to be in 'a confrontational situation'.

As Dembele didn't react it was seen as a friendly chat.

I've thought that was a bit of a silly rule
Fair play to Dembele, I'm sure some players would have taken advantage and complained to the ref that they were racially or otherwise verbally abused, even though they hadn't been...Ref then no option but to look at replay, can't dispute if anything was said, and send the player off?

Pity a certain person's mother didn't put her hand over her chuff.