You are correct to a degree. Except even the state legislature when it decided to allow mail in voting in 2019 broke the rules as it went against their state constitution. They needed to change their constitution which had the voting rules in it; which requires a 2/3 vote, then a vote/referendum put to the voters, etc.
So the legislature broke the rules, and then the state court added all of the other changes which also broke the rules.
So when there’s an utter mess like that, in my judgment it seems totally appropriate for a harmed party such as another State to bring suit - because there were legitimate matters of law.
If I understand correctly, the state of Texas is not quibbling over which type of machine or tabulator/enumerator to use, but rather a matter of law.
So I don’t see it as one state meddling with another state’s rights or process, but rather requiring that state to abide by its own rules - and requesting redress when those rules were violated.
We are a nation of laws, not a nation of whimsical secret negotiations. Or at least, we OUGHT to be!