One commenter on X, who said their partner was on the Delta flight, reported that it “smelled horrible” and that vanilla-scented disinfectant used to mask the odor “only made it smell like vanilla s---.”
Code shares are weird and if you don't know about them you should look them up - what I'm saying is that code shares don't get split between two major carriers as far as I know.
So you'll never have a flight that is both United and Delta ticketed. But you'll often have a flight that is United and ANA ticketed. Or Delta and ItalAir
Diverting is really expensive for the airlines, so you know they only do it if there is no other way. So it can't just have been a bit of a bad smell...
Some smells mask smell of shit better. There's even an old-school method of lighting and putting out the match after you did number 2: sulfur smoke kills the shittiness of ahit smell somehow