Ah. You're on Void. Indeed, it does look like Void doesn't have an implementation of systemd-tmpfiles. Here is the file from my system, it's only creating a single symlink, which is basically the same thing you are doing:

[moonpie@nefertem home-manager]$ cat /etc/tmpfiles.d/non-nixos-gpu.conf 
L+ /run/opengl-driver - - - - /nix/store/2r6vmqh1vkivsrmr6n1vh1862i2qndqz-non-nixos-gpu

Oops, the below is for artix. I was nosy but not nosy enough and I saw a comment where you mentioning you used void after you wrote all of this. I'm gonna leave it here since it was an enjoyable rabbit hole.

I did some digging, and it looks like it is possible to use systemd-tmpfiles outside of systemd. It's just a binary that creates files/directories/symlinks based on a format, so it's pretty easy to extract, or reimplement entirely.

Artix has their own, an extraction from the systemd package: https://packages.artixlinux.org/packages/system/x86_64/etmpfiles/

It looks like that package is required by the base package, which is the core package set declaring the minimal dependencies needed to have a working system, so it's highly likely you have it installed.

I think it automatically starts too, but I can't find a hard source for this. Based on this page: https://wiki.artixlinux.org/Main/Migration#Download_the_Artix_packages it looks like etmpfiles is needed before you select/install the init system, which suggests that. This reddit post, where a user has an issue where etmpfiles isn't installed, so their service manager complains and dies.

If you have a service named something related, like tmpfile or the like, then it does autostart.