Then what is your definition of a power user?

Someone who, like the GP says, uses third party clients and ad blockers, who is very technically knowledgable, who may even use Reddit's API directly themselves, or write tools/clients for Reddit.

Contributing to Reddit's content doesn't require any of that, and I'd wager the overwhelming majority of Reddit users don't do any of that but do contribute most of Reddit's content.

Just because you're a content creator doesn't mean you're a power user. The concepts are mostly orthogonal. Though if you're a mod you probably use some mod tools, which makes you a bit of a power user by definition. But mods are a tiny minority of Reddit users and don't contribute most of Reddit's content.

the vast majority of users doesn't comment, let alone post.

They don't, but those that do are just ordinary users.

Power users are a tiny minority, and most of them don't post either.