> And why do they not include the sea-level component, similar to that which I discovered in another thread. Is that because it is not important? Or that they have not considered it?

I think it's for historical reasons. The MEI was first developed about 20 years ago, and they didn't have long-term, gridded, sea surface height satellite data products like TOPEX/Poseidon. It's probably worth considering, though, as an indicator of subsurface heat content.