I think the WIP that line's from is dead, but I may drop the fun bits in a more current-canon-compliant continuity? The setup in the original version, anyway, is it's a few days after Marinette and Adrien learned each other's identities in circumstances that led Sabine and Tom to conclude Marinette and Adrien were secretly dating for a while and had probably fucked in Marinette's bed the previous night. Marinette and Adrien did not disabuse them of that notion—which is to say, the shovel talk has already been had. They did however beg that no one let Gabriel find out.

Sabine did not get a word in edgewise after agreeing to play along with Marinette, and when Marinette hung up, Sabine stared at the phone for a bit and then died laughing and then went to tell Tom how clever their daughter is.