The timer should work when using non-demon-bane weapons. It currently "works" by accident with demon bane.
https://oldschool.runescape.wiki/w/Doom_of_Mokhaiotl/Strategies#Demonic_larvae
Demonic larvae have 2 Hitpoints and can only be one-shot by demonbane weapons, multi-hit weapons, and the Eye of Ayak; any other weapon will only deal 1 damage, requiring two attacks to kill one. They may be attacked on attack cooldown: Non-demonbane attacks incur the weapon's attack delay afterwards, whereas demonbane attacks and the Eye of Ayak do not affect your original attack delay. Fast non-demonbane weapons (e.g. a blowpipe) may be used to overwrite the original attack cooldown, allowing for faster attacks on the Doom.
The timer should work when using non-demon-bane weapons. It currently "works" by accident with demon bane.
https://oldschool.runescape.wiki/w/Doom_of_Mokhaiotl/Strategies#Demonic_larvae