From my understanding, in which they told me, is that...
They have diff ammo for walk ins and different set of ammo for people on the list. By different, I mean, it could be the same type, for example, 147g lawman 9mm. If they bring in 1 case of a thousand rounds, half will be for walk ins and half for the people on the list.
Thats just an example.
Its giving a chance for new shooters to be able to purchase a gun and actually be able to get ammo. Id be pissed if I bought a 1k sig and after the purchase I hear " oops sorry , you gotta wait 10 months for ammo."
Then I'm gonna generously assume, if they only allow "walk in ammo sales " exclusively to people who purchase a new gun then that would be unfair. On the other hand it would be a great selling point. Lol.

I know they put walk in ammo sales for ammo that doesnt move quickly. Whether it be on the expensive side (pre covid 50 rounds gdhp 9mm was 25 bucks or so, now its close to 40 bucks) or just ammo no one is looking for.
I'm on the list as well. For several calibers. But I'm picky so I know I have to wait. For example, I was only looking for 223/556 62g or higher soft points. Took them bout 10 months but they called and I bought a few hundred.
I think they have great customer service. Once I saw this old guy with white hair, skinny, very pidgeon accent come in. Apparently, he never got the memo that rifle and pistol ammo has the same rules, once you make a purchase, you cant come back 2 weeks later asking for the rest of the max ammo amount thats allowed. He complained he purchased 9mm ammo (bought the whole 1000 rounds at once) and thought he could sparsely buy rifle rounds until he met the max amount. Iirc, he bought 4 boxes of 77g otm black hills 2 weeks prior to that day, and while talking to rich, he sorta demanded for the remaining 6 boxes. I really wanted to punch him, dont fucking demand shit like that. Fuckin Karen. Or Ken. Whatever you wanna call those type of people Lol. The lady let it slide and allowed him to buy a couple more boxes and put him back on the list.
They always leave voicemails for me. Sorry they didn't in any of your guy's case.
I have no affiliation with them. Just my experience.