Probably referring to the definition, not the difficulty.

Yes, NP-complete is a subset of NP-hard. Grandparent states that NP-completeness only applies to decision problems (which is correct), but seems to think that NP-hardness applies to optimization problems (which is incorrect; NP-hard too is a class of decision problems).

didn't mean to imply "only" as a less-than comparison -- just a distinction.