There's two sets of plaintiffs in the suit, the state plaintiffs and the private plaintiffs.
The state plaintiffs alleged standing on the basis that they have an interest in making sure their citizens have a "right to listen", and this was yeeted almost perfunctorily (it's like two pages of the decision). The states' lack of standing is not the major holding of this decision.
The private plaintiffs alleged standing on the basis that they were censored by the platforms as a result of the government's encouragement, and they were yeeted specifically because they failed to demonstrate any traceability of their censorship from the government's action.
Yes, I read the ~38 page majority opinion. (I gave up on the dissent as not being worth my time).