I made it implicit, but forgot to say out loud: While system gestures makes the open-drawer-edge-gesture worse to the point of unusable, I think that looking back it wasn't that good to begin with. Which is why I think Material Design never officially supported it in the first place, way before gesture navigation was a thing.

with a back button, it's always in the same corner; with a back gesture what happens when the phone is rotated? (i genuinely don't know) does it stay on the side it was on, or does it move so it's now on the left? wouldn't that make it very hard to reach with one hand?

The pill-thingy is anchored to the bottom of the screen, so basically it always point to the ground, the back gesture is on the side. It isn't uncomfortable because it matches the easiest point to reach, which is the side of the screen relative to the user, not the device. This very old image shows the most reachable areas of a screen:

https://miro.medium.com/v2/resize:fit:640/format:webp/1*9IDju9_qkRNyKpueLBhkgg.png

yes, but it doesn't extend beyond the confines of the drawer. in my head it's more like the drawer is always there, and you can trigger it with the hamburger, or "drag" it into view manually

That is fair. However, the drawer isn't visibly always there.

i mean yeah, that's the very point of material design. the whole spec goes on and on about the information conveyed by implied depth

but even before that, drawers and menus usually had some type of drop shadow to imply depth (practical skeuomorphism at work^!^)

It is a bit funny that the design system designed to resemble real life materials didn't really accept that metaphor. Besides, my point was that the gesture itself was implying physicality, you could move the content to swap between tabs, but the gesture was 1:1 with the content. With a drawer you weren't moving the drawer into view, you were reaching a glass handle that was glued to the drawer. Either that, or you were reaching into the realm beyond the screen to bring it on. That is something that I never quite liked for app gestures, because not only is it implied that things exist outside the screen(which is fine), but that you can somehow reach for them.

but a diagonal swipe does something else

Erm, there is no diagonal swipes, at least officially from Google. You just swipe perpendicular to the edge of the screen. So from the bottom edge you swipe up, from the left edge you swipe right, right edge to the left