they clogged the stairs with dead walkers, then .they just started shooting them all. In the scene where mullet boy is sitting on his bed, after the walkers had taken over the 1st floor of the place, you can hear continuous firing going on.
Yeah - the ammo thing is a glitch, tho--- I mean, mullet-boy said he can make more ammo for Negan, but that couldn't have happened by the time the saviors got out and are now putting the siege on, with guns and ammo in their possession....after they supposedly used up most of it to get rid of all the walkers....
I think the original plan (ricks etc) was to blockade them with walkers and wait them out and negotiate a surrender, with Negan being the only one killed.

But yeah - how did Carl let himself get bit after all these years :/