How sure are you? In CA we don't even know if those who placed a measure on the ballot have standing before the court if our governor and AG refuse to defend an initiative the voters passed.
It's already happened (at least once) that I can remember.
Back in the early 1980s, Congress was granted standing to appeal a case to the Supreme Court - a case that had been abandoned by the executive branch, not entirely different than this one. The case was Immigration and Naturalization Service v. Chadha, 462 U.S. 919 (1983).
The facts of that case aren't relevant to this case, but the issue of standing for Congress exists, nonetheless.