While the Diablo is a car that ran in the series, and thus is eligible for SuperGT/JGTC races, in its stock trim it does not fit the class. I believe in the description it mentions that it ran in several different series. In order for it to run with GT500s it needs to be limited.
As it arrives it is heavier by 100kg than the others, but that is not enough to offset the fact that it has 100 more horsepower than any of them as well. It's basically a full GT1 car originally, and its PP reflects this - roughly equivalent to a C5R. I don't know how much limiter/ballast it needs to be properly competitive with the other GT500s(I'm sure someone does) but straight out of the dealer it will surely wipe the floor with them.
This might be part of why some people online won't let it compete. Without a hp/pp limit it wouldn't be fair.