The case cited touched on both state and federal causes of action. The TDP filed the case in state court; the RPT removed this to federal court. (From the case cited: “The TDP filed this suit in Texas state court on June 8, 2006, seeking declaratory and injunctive relief. The RPT removed the case to federal court, where on June 26, 2006, the court held a hearing on the merits.”) The federal court ruled on issues of both TX state law AND federal Constitutional law. Under the state law of TX, under the specific conditions mentioned, both the RPT AND candidates for POTUS from the RPT have standing to challenge a candidate who is ineligible to appear on the ballot. (Because of the concomitant issue of Constitutional qualifications, the TDP could have brought this case in federal court instead of state court, in the first place.)
The case cited is dated 2006.