they can with airborne. in 2v2s and higher youll want your teammate(s) to help you as well, since the flak HQ is big objective that you and your team should try very hard to destroy.
if you dont go airborne, and LT tier, a jackson replaces the AT gun. obviously your attack on the flak HQ will be delayed since the jackson comes out later than an AT gun. if you go captain tier, fighting pits + enough riflemen in green cover are a big enough deterrent in dealing with enemy infantry.
in addition its helpful to have stolen paks or mg42s obviously.
See how much effort it takes to destroy this building? Not to mention, the map has a huge influence on how Allied players can handle this situation. Add this to compounding problem of committing 25% of your troops for this task while Axis player is free to roam the map and do all kinds of damage (map control, ambulance hunt & flanks). Lastly, the areal denial on this truck makes it difficult to pull off mid game (he/she can sit on a fuel point until then). The whole point of this truck is to keep opponents from cutting off OKW (slap in the face for COH2 tactical play).