I just downloaded 2.0.0 and looked inside the zip for nosiness reasons. data.lua has a size of zero bytes.
Is the mod just some sort of "tag" for the main mod (which it oddly enough doesn't seem to depend on, btw)?

Yes, exactly. The main mod checks if this mod is enabled and changes some behavior if it is.

I suggest adding "all chests" as an option to the existing dropdown in the settings of the main mod.
That's way less confusing for users like me.

Problem with that would be if you enabled all merged chest types it might require too much memory/time and they would would be forces to reset mod setting. Same thing goes for WideChestsUnlimited mod.

Having separate mod allows you to disable this mod, disabling the functionality.