If the opening crawl for ANH is anything to go by, Thrawn is going to have a field day on these Rebels...
If the ´first victory´ mentioned in the crawl is the battle in Rogue One (and we can assume it is) then it could indeed mean there is no other major victory before...
But then... what happens to Thrawn? He isn´t in Rogue One, he isn´t in A New Hope.
Will he die through treason, defeat? Or get ´banished´ to the Unknown Regions?
I'm going to guess banished to the outer rim to allow for Timothy Zahn's original series to still have some merit.