It's not factual, but I think it's implied in the Metallic battle. Goku is very impressed by Metallic after fighting Roshi who also demonstrated incredible durability, so I don't think Goku is only commenting on Metallic's durability when he compliments him.
apter: 59, P8.2-3
Context: talking about Metallic’s missile
Goku: “Wh-what was the technique just now? Amazin’, I’ve never met a guy like this before…! He fired something that looked like a Kamehameha from his mouth!”
You can argue that he is only referring to Metallic's techniques, as Goku then gets serious and tries to take out Metallic with his own KHH. Goku then says that he wasn't using his full power, so I tend to think something like: MT Goku > 21st Goku/initial MT = Roshi = Metallic