There is a difference between signing on as a plantiff, and filing a amicus curiae brief in support of one side of a case. As far as I know, Texas is the plantiff and the other states are just filing in support of Texas.
No, some are actually intervening.
That's because if they joined as plaintiffs they would have to state a complaint and the defendants would have to be given time to respond. Filing amici allows them to state whatever they want and the defense doesn't have to respond so there is no need to delay. These filings are serious. Don't doubt it.