Are these surfaces supposed to be flat? As shown here, mine aren’t.

Yes. They are supposed to be flat. Find another adapter or a shop that has a post mount facing tool.

Is that really a thing? It seems weirdly specific

Edit: I'll be darned

It is really a thing, but its really for facing post mounts on a frame, not an adapter that can be easily replaced. I'm not sure the cutter will be large enough to face the entire rectangular surface of the adapter. https://www.parktool.com/product/dis...ing-set-dt-5-2

If it were mine, I would just clamp the adapter in my mill vise and mill it flat. I don't expect most people to have a machine shop in their garage.

To the OP, yeah, those faces should be more than 50% flat. 100% isn't necessary, I'd consider 70% to be a fair target.

The Park tool you've linked is likely the better tool for the job than your home mill. It's important that the post-mount flats are perfectly perpendicular to the rear axle, not to the welded disc tab.

Since this is a Salsa Alternator dropout, it seems very likely I could take it off the bike and accomplish the facing with a sanding block.

However I'm not gonna. I found this when I took the caliper off and it made me curious. But I took the caliper off because I'm chasing a noise coming from the rear. And... This wasn't it. It's becoming more and more inescapable that it's the hub bearings

Interestingly enough, one of the pictures at your link actually show the tool being used for facing an adapter.