I don't know. Looking at the setlist, it was a hugely demanding show to sing. But he sounded great at the RAH concert, even though that was a tough setlist as well. I think the vocals in the studio haven't been showing the decline, however. Yes, there aren't as many high notes maybe as 5-10 years ago. But his technique has actually greatly improved since he quit SYL, and the male voice gets lower with age anyway, especially if you scream like Devin does. At recent live shows when I've seen him, his vocals were mostly very good.
I think this is just due to a number of factors. He's mentioned on Twitter that this year has been one of his most demanding so far with all the touring and travelling for interviews and promoting gear and stuff. Moreover, the other two full OM performances that he's done so far didn't have a 1 hour show in advance already. Lastly, he's mentioned that doing OM in full like this is pretty much an emotional rollercoaster, and this time he even had the original bassist come on and play the whole thing with him, and he had the orchestra play the first set with the DTP, which can also be very stressful. Being stressed out a bit can greatly strain your voice when you have to sing.