I can’t read the article because paywall, but I assume this ruling is based on the fact that political parties are “technically” private entities and therefore primaries are “technically” legally-meaningless private contests that said parties can run however they want.
Which is all bullshit, of course – if political parties were truly private then they wouldn’t get any support from the state whatsoever to run their fake elections, for starters – but just goes to show how their corruption and power lets them (as de-facto government entities) get away without accountability or oversight.