Why do you think so?
I'm glad you asked.
Stallone might not have been up on a stage wearing nothing but a thong, but that doesn't make him any less of a bodybuilder. Being in his 60's he's spent more than half of his life building his body and is in better shape than most, one has to remember that Stallone never was genetically gifted for bodybuilding... he never spent all day lifting and eating... he had a fantastic movie career, yet he was able to build up his tremendeous physique!
He didn't start bodybuilding seriously until he was well into his 30's.
He was in shape for months and months on movie sets... compare that with a bodybuilder who just peak a few times a year!
Imagine the hectic schedule of Stallone? And still he's able to keep in that awesome shape!! That's mindblowing!!