But that makes no sense.
Nope

It's weird, Gibson was basically the
only writer who attempted to explain how there can believably be Aliens on the Sulaco after the second film - by having some goo the Queen stuck in Bishop when she impaled him develop into an egg... and then he went and stuck in a couple of magic adults for the lulz.
He fixed it in his second draft by just having the egg, but still. Amazing how he, Red and Ward - and then the shooting script - all had Aliens running amuck on the Sulaco without any explanation.
I just now noticed that Giler and Hill are given the story credit in those scripts, even though the screenplays were written by Gibson.
As Kiramid says, I believe Gibson worked from a loose framework given to him by the producers.