You've got to wonder.
Five of the attackers may have had ties to America. They might even be American citizens under our loosey-goosey citizenship on the cheap policies. But they certainly aren't Americans.
I'm not defending the decision to put everyone in one aircraft. That was stupid. I would not be surprised to learn that whomever made that call also had a role in leaking the details of the mission beforehand. IOW, they were "set up".
However, the "slow moving transport helicopter", the Chinook, has a Cruise Speed of 130kts while the Blackhawk is listed at 150kt Cruise Speed. The US "attack" helos do not carry troops. The maximum speed for the Chinook and for the Blackhawk are both listed at 170 kts.
While the Chinook is obviously a larger target, differences in the speeds are meaningless when compared to RPG, MANPAD or AAA fire velocities.
BTW, CH-53D pilots used to "dogfight" with their next door neighbor Cobras. The Cobras had a higher VNE and could go faster in a dive, but not straight and level, in a climb or at altitude. Apaches are faster, though.