When a mob is punched and jumps into lava above its head it will die and drop XP, indicating a player kill. It is also possible to get multiple and rare drops if using a Fortune enchanted weapon.
However the statistic for the mob is not updated to show the kill, which would seem to indicate that this is not a player kill.
In the screenshots the zombie is punched into the lava, dies, and drops XP, but the stat (which I can't screenshot unfortunately) doesn't update.
Kill a creeper, observe the objective go up by 1
Punch the summoned creeper into lava
Observe xp and items drop without the objective going up