With out a doubt it's pretty safe to say its GGPO.
Original tweet was like 95% confirmation, other 5% got further confirmed by the CM and Derek and Boon all kinda saying its true with out actually saying it.
If you still want to be pessimistic like me, just think of all the ways they could screw up the implementation.
Online could be broken for weeks, constantly teleporting, even more desyncs, its not even a full netcode swap, but just an optional mode that is exclusive to private match.
But man, if it works, I will love you forever.
And even if it doesn't, we know they are actually willing to implement it, and it says good things about Injustice 2/WWE Immortals console edition.