This was the wingers' answer to our Hail Mary a few years ago to use weighted census projections instead of raw data for seat allotments. The theory being that a lot of people are missed (the homeless, people who are not the sort of guys who have a post box, etc).
Same idea as voter suppression laws: use a scintilla of rationale to try to rig the system to favor your side. Unfortunately, the orcs have been getting away with those, but a sane Court will probably give them the old heave ho (and hopefully reinstitute the full VRA). Mortality has consequences, Nino.
As I read it, states MAY use total population as opposed to registered voters. So, in effect, the SCOTUS deferred to the state legislatures. Good.