Ohh because the devs wrote means it makes sense jeez louise they shoudn't even be able to scratch there t-b51 with turd service rifles
What the Devs right is canon, regardless of what we think. What you can do is find a plausable explaination to fit the evidence, especially if facts about the battle are pretty much non-existant. Maybe their was a heavy Ranger presence at the battle, Helios is described by the BoS and by the new NCR garrison as very indefensible, maybe they kitted their men with pulse grenades for this important task, maybe the Brotherhood ran out of ammo for all their fancy [censored].
Finding a explaination to suit your view is better than pissing and moaning about it not being fair. There are no accounts of the battle other than the Brotherhood ultimately lost and the NCR heavily outnumbered them. You want to defend the BoS then do so, don't complain.
EDIT: You open up at any piece of armour with 15 rifles and it will be penetrated. Power Armour is tough, but if you unload a minigun into it - of 5mm ammo no less - then you would expect it to falter.