If For my Battle Brothers is played, Can they remove a Genestealer from a Swarm that has Power Field played upon it, as 'Attack' isn't specifically mentioned?

No.

The card is an attack.

It's just a wording issue, and should read 'slay 1 genestealer'.

Do we have an official answer on this?

Power Field actually doesn't say anything about preventing an attack, it just prevents any GS in the chosen swarm from being slain. So any effects, attack or otherwise that attempt to slay GS in the chosen swarm fails.

Though I could see an argument for Intimidation, since shuffling back into the blip pile isn't really the same as slaying. (Then again, is "discarding" the same as slaying?)