I think it's worth noting here that the ruling doesn't forbid states from drawing lines based on voting-eligible population, but rather SCOTUS declined to force states to do that. They didn't say drawing lines by voting-eligible population was constitutional, either. They neglected to address that issue entirely.
I could have misinterpreted this a bit, but I think it is clear that this ruling doesn't bar states from drawing districts like that.
Does anyone else with some legal bonafides have insight into the specifics of this? Possibly someone whose username begins with Tor and ends with ie?
That's correct. There were really three positions being argued in this case. The plaintiffs argued that the "one person, one vote" rule
requires apportionment based on voter-eligible population rather than total population. Texas argued that "one person, one vote" does not mandate either rule and states are free to decide which to use. The United States argued that "one person, one vote" requires use of total population and forbids the use of voter-eligible population instead. The Court rejected the plaintiff's argument but didn't actually adopt either of Texas or the United States's view. The Court just said that "one person, one vote" permits the use of total population, and the Court did not reach the question of whether the Constitution forbids or permits the use of voter-eligible population instead.
The Supreme Court, and other federal courts may only hear actual "cases". This particular case was a challenge to the Texas redistricting which was based on equal total population, not equal voter population. In order for the Supreme Court to decide on the constitutionality of voter-only apportionment, a State would have to actually adopt the scheme and draw maps under the plan, and a legal challenge would have to work its way through the Fed Courts. So voter-only apportionment may be flat out unconstitutional, or OK. But in this case, the Supreme Court unanimously believes that it is permissible for States to base maps on equal population. Thomas concurred on this, but called the "one person, one vote" test a stupid fiction that should be eliminated. Alito also concurred on the outcome, but took issue with certain history presented in the majority opinion.
I don't think this is quite true. Here we had a justiciable Article III case where the issue was whether or not Texas's districting plan violated the "one person, one vote" rule articulated by the Supreme Court. The Court could definitely have reached the question of whether that rule permitted use of voter-eligible population if a majority of the Court had wanted to. The question was what the "one person, one vote" rule actually means. Plaintiff argued that Texas must redraw its lines because the rule is X. The Court said "No, because the rule is not X." But the Court could just as easily have responded, "No, because the rule is Y."
