I think I found the source of the problem.
There’s currently a bug with Arch Linux’s packaged android-tools that causes the exact error that terminates fastboot: 34.0.4-1 breaks fastboot (#4) · Issues · Arch Linux / Packaging / Packages / android-tools · GitLab
Checks out because I ran this on Arch. Looks like Arch is screwing people over right now.
I’m unsure of how this is supposed to play out for FP. It’s not directly their fault, but neither is it the end user’s…? FP could’ve included minimal android tools with their firmware package to prevent this (and I’m not sure why they included them for Mac & Windows but not Linux), however it seems not even they caught this issue.