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.

i mean, i'd say it's better than nothing. otherwise one has to reach all the way up to the top to press the hamburger menu

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:

that doesn't seem hugely ergonomic - i can hardly reach the bottom middle of my phone, let alone the left edge

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.

well yeah, it's this exactly. your finger starts from off-screen, where the drawer is currently hanging out, and drag it on-screen. almost exactly the same as the notification drawer, the control centre on ios, or that stupid "charms" thingy on win8

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

i thought that was how one opened nav drawers with gestures enabled? a diagonal swipe; or swipe, wait, then swipe a bit more (which i'm not even going to go into how awful that is). including in official google apps?