As a bit of an aside, I would never accept a PR like this, and would always update $large_vendored_dependency myself. This is unreviewable, and trivial to insert any backdoor (unless you go through the motions of updating it yourself and diffing, at which point the PR becomes superfluous). I'd be wary even from a well-known author unless I knew them personally on some level (real-life or via internet). Not that I wouldn't trust them, but people's machines or accounts can get compromised, people can have psychotic episodes, things like that. At the very least I'd like to have some out-of-band "is this really you?" signal.
This is how I once inserted a joke in one of our (private) repos that would randomly send cryptic messages to our chat channel. This was pretty harmless and just a joke (there's some context that made it funny), but it took them years to find it – and that was only because I told them after I quit.
That said, looking at the GitHub account I'd be surprised if there's anything nefarious going on here. Probably just someone using your repo, seeing it's outdated, and updating it.
The (most?) popular SQLite driver for Go often gets PRs to update the SQLite C amalgamation, which the owner politely declines (and I appreciate him for that stance, and for taking on the maintenance burden it brings).
In this case, the project is using Git submodules for its vendored dependencies, so you can trivially cryptographically verify that they have vendored the correct dependency just by checking the commit hash. It looks really crazy on Github but in most git clients it will just display the commit hash change.
This is how I once inserted a joke in one of our (private) repos that would randomly send cryptic messages to our chat channel. This was pretty harmless and just a joke (there's some context that made it funny), but it took them years to find it – and that was only because I told them after I quit.
That said, looking at the GitHub account I'd be surprised if there's anything nefarious going on here. Probably just someone using your repo, seeing it's outdated, and updating it.