Does the reduction to evolution from absorbing pollution apply to modded buildings too, or just trees?

No, unlikely. If the modded building is using Lua code to remove pollution then that is invisible to other mods, unless the mod makes it visible. NGE finds how much has been absorbed by trees by looking at the statistics page (production statistics > pollution). If the modded building also modifies the pollution statistics (to the tree or tile proxy entity in the stats page) then the answer is yes, it will apply.

If the modded building are well written then the pollution absorbtion should be visible in the pollution stats window. If they use another item than trees or tiles then I could add that to the list of items that count as evolution reducing sources when I know what it is.