SideQuest changes the render texture size - which is equivalent to eye-buffer size, and actually is the same as resolution.
Per Meta's official documentation:
This is the resolution of the texture to which the app is rendering. The default for Meta Quest is 1216x1344.
Confusing technical details aside: This has nothing to do with in-game textures, and everything to do with resolution.
This setting is already in Quest Game Tuner, as "Render Resolution" - which more clearly describes what it actually does