Bearing in mind the Saudis launched airstrikes against ISIS in Syria themselves a couple of months later, it's not really plausible they want[ed] to protect ISIS as opposed to avoid the US stepping in to stabilise the Iraqi government (or bombing Sunni cities)

ISIS being Sunnis actually makes them more dangerous to the House of Saud, since they represent a challenge to their legitimacy as rulers rather than their population's natural enemy. Similarly, the US damaging the ISIS economy and prestige actually strengthens Saudi Arabia's position as the benefactor-of-choice for aspirant Sunni militants.

I have a friend from the region (Aleppo, to be precise). He claims that Saudi airstrikes actually hit the groups fighting ISIS, and not ISIS. As I haven't followed the news as closely as he does, I'm inclined to take his read on it.