According to baseball savant these metrics are 100% accurate. That’s roughly average rise and 3 mph above average. This chart grades it as plus, and since I have no idea how they weight each component I can’t really disagree, but I think I’d grade it 65.
The whiff rate he got in the game was likely some combination of terrible batters seeing a plus or better heater for the first time, and the fact Didier seemed to locate it well a lot of the time.
So the fastball is really nice, but the hyperbole of comparing it to anything resembling strider's 80 grade pitch is silly. We can be happy to see Didier improve his fastball into a weapon without going full posi derptard.