Makes sense. So, I just found rebuild-detector thanks to @anon26269396. 
Another way the rebuilds can be done is to just reinstall while making sure to select “A” for “All” when it asks if you want to clean build.
Example:
checkrebuild
foreign emote
foreign gimp-devel-noconflict
foreign python-manimpango
foreign xnviewmp-system-libs
yay -S emote gimp-devel-noconflict python-manimpango xnviewmp-system-libs
==> Packages to cleanBuild?
==> [N]one [A]ll [Ab]ort [I]nstalled [No]tInstalled or (1 2 3, 1-3, ^4)
==> A
checkrebuild
Of course, this will take varying amounts of time depending on how many packages you have and the size of packages that need to be rebuilt. So, I guess people can do them one-by-one depending on how much time they have available.
EDIT: Running checkrebuild again shows me gimp and xnviewmp again. Maybe I need to reboot/logout? Will find out later.
Think I just found out why:
But also, maybe not because gimp definitely gets built as it alone took about 80% of the rebuild time for the packages I reinstalled/rebuilt. About 15 minutes total.
Note: According to the information on the GitHub page, it kinda seems like you may not need to rebuild certain packages? But I’m not sure.