This is incorrect / misrepresented / explained incorrectly.
The PROVISIONERS and SETTLERS make use of one round of ammo, NOT your followers.
You basically give them one round, they have infinite ammo. I give 12 rounds to monitor frequently.
Even after getting hammered at Dalton Farm, the amount they have never changes.
I own all the settlements in the Commonwealth and all 4 in Far Harbor, there has been no change about this trick.
Example, even in Far Harbor, Nick blows through ammo like it's candy, while my provisoners and settlers never use theirs up.
All my provisioners use maxed out miniguns, with 5mm ammo. Plus I give them one grenade.
They cause shitstorms everywhere they get attacked, just like in the Commonwealth.
For all my followers I give them a Legendary weapon and around 200 rounds.
When they use it up, they go to the Melee weapon I gave them.