Sure I can answer your questions

(1) it’s a fairly new product and a point of emphasis for the new 1x product as the shirt cage RD’s use up to 33t cassettes
(2) there are really only 2 major manufacturers of components in the US. Sram is one. It’s not like he’ being asked to know something about a bike-brand-specific issue.
(3) of course he shouldn’t pretend or lie. But, better to know then not know. And much better to know where to look for the answer if you don’t know. Like, “ Hey, Shopper, I don’t know off the top off my head, but give me two minutes and I can find you the answer.” Or “I’m tied up right now, but I know where to look and will have the answer to you tomorrow.”
(4) I disagree with the premise that a Mechanic’s value proposition is limited to wrenching. I expect them to be able to offer insight/expertise on how to build a bike with compatible components, what works well/not well etc. I recently had my mechanic do a build and they offered me all sorts of advice on things like disc brake size, valve extender choice, use of grease in certain bike parts, and some issues that were specific to my bike (the proprietary stem and handlebar).

Do a Google search for "SRAM Red AXS rear derailleur".

Click on the first result.

Click in Specifications.

It says Max Tooth 33.

Sure. Now what is cassette ratio on the bike that the OP is ordering?

OP believes that the bike comes with a 28 tooth large cog.

Specialized website says that it comes with a 33 tooth large cog.

Fireable? I’m not saying the mechanic should be lose his job. I’m just saying I’m not gonna be using him anymore.

I never said that the mechanic should be fired.

They should be able to figure out that the rear derailleur can handle a 33 tooth cassette in about 10 seconds.

That’s what a firable offense means in this context. You aren’t using his services. Obviously the customer can’t actually fire him from his job. All Based on a tiny snippet of a conversation thats being relayed secondhand.