He could've still boarded the second dropship, taken it to the Sulaco, quickly hide the egg and then bolt back to the colony to pick up the survivors. It's defenitely a stretch and defenitely sound beyond time-optimistic with a huge window of failure, which would directly have Bishop responsible for knowingly prioritizing alien life over human life, but still... He is a company asset.
Again, I don't like that explanation at all as I rather just interpret the opening of A3 as a semi-awake, semi-cryosleep dreamlike nightmare blur of what actually happened. To me it's a mystery rather than a plot hole. It doesn't affect the rest of the movie for me other than setting the tone (it's a great opening sequence with or without the egg).