Nobody believed that Wawrinka would do what he did back in 2014 at the AO. Had he even won a single set against Nadal before that? Maybe one, or none (I'm not sure). Something changed then, and I think it doesn't have entirely to do with Nadal being injured (which he was).
Yes, I am reasonably concerned about tomorrow's match. Thiem might have had the superior statistics for the clay season, but it's Wawrinka who has shown to be able to defeat Nadal in a slam final before, and after his performance yesterday against Murray, it can reasonably be predicted that he will come out with a very aggressive mentality. That's exactly the only way anyone can beat a Nadal in this form at the French Open, and if things work for Wawrinka and he can keep a healthy Winner/UE ratio, the match will be very difficult for Nadal.
But I hope Nadal wins this, obviously! You are right, it has been a long dark era for Nadal fans since late 2014. He had gone through tough times before, but never like this. It's hard to believe how he has risen from his own ashes once more!