I havn't actually played any games using Air units yet, but I just wanted to get some clarity as to what this part meant.

"When a hoverer performs a SUSTAINED ATTACK action, extra care must be taken to keep it as stabilized as possible to increase its accuracy. This also makes the hoverer more vulnerable to attacks from other units. After a hoverer performs a SUSTAINED ATTACK action, until the hoverer’s next activation, all other enemy units can target the hoverer with a SUSTAINED ATTACK action of their own. An enemy unit can only perform this SUSTAINED ATTACK
as part of its own activation, and must be within range of the hoverer and use a weapon that can target its armor value to do so."

Does the "action of their own" and "own activation" imply that you can do a normal attack/sustain attack to a hoverer with any unit? (by normal i mean roll, then re-roll misses)

No. Shots at aircraft can only be sustained but you don't get the re-roll due to the planes speed. Because a chopper can stay where it is to do a sustained attack it means its not moving, so other units can shoot at it as normal shooting.