It’s the 20th anniversary of TPM, not AOTC though.
There are plenty of actors there whose films don’t have an anniversary or aren’t about to come out.
Besides, there’s the obvious: if Hayden’s in IX (which I doubt), they wouldn’t announce it at celebration or anywhere else, it would be a surprise in the theater.
Hayden has only done two Celebrations, 2002 and 2017. I think one of the reasons Lucasfilm got him to come back in 2017 was to test and see how fans will react to him.
Or maybe it was the 40th anniversary and they brought back as many people as they could and Hayden realized he liked it and could also make a fuck ton of money selling pictures and autographs.