NSO will very likely not be able to _prove_ their version of events to the level needed to prove "fraud on the court"; that challenge was just thrown out there to ratchet up the rhetoric. The "Hail Mary" that they're trying for is for the court to dismiss with prejudice (so Facebook loses permanently), but that's a very very long shot!
If NSO's claims are factually correct and the service of the subpoena was actually deficient, then the most likely outcome will be that the judgment will be vacated and remanded for further proceedings and the "did Facebook's attorneys knowingly perpetrate a fraud when they claimed that the subpoena had been served?" will be mooted.
Note that the NSO argument is that allegedly 2 days prior to the hearing, Facebook's Israeli attorneys were informed that service was not complete, which seems awfully close, and it seems very likely that Facebook would simply argue that they believed that (a) the subpoena had been served, and/or (b) the suggestion that it may have been deficient was itself deficient (i.e. what we have here is two lawyers arguing about the law, which is normal and what courts are for).
Also note that NSO is also arguing that they are operating under Israeli government sponsorship in producing their malware, and gosh, the Israeli government is the party asserting that the service of the subpoena is incomplete. Golly.
Even from just reading NSO's filing, they acknowledge that they're well aware of the case and it's calendar, and that they're claiming that they aren't subject to the court's jurisdiction. Now, point to ponder: is a California court more or less likely to view a party's filinigs in a favorable light if they make their objections known in a timeley manner, or if they wait until after the court has wasted time issuing a judgment of default?
The simplest (and cheapest) approach for NSO would have been to have an attorney show up in the courtroom, note that they arent waiving jurisdiction claims, but by the way Facebook hasn't managed to serve them yet, so see you in 90 days (or whatever).
To be honest, this looks like NSO is trying to drag things out as long as possible, which is suggestive of, shall we say, a certain awareness of risk...