“Don’t pretend to be somebody you’re not” means that you can use a pseudonym, but you can’t impersonate a real person or falsely claim to be in some class of people. You don’t have to disclose who you are, but you can’t try to mislead people about who’s speaking.

• You can call yourself “John Doe” (unless there’s already somebody else in the discussion using that name).
• You can’t call yourself, say, “Prime Minister Stephen Harper, Ottawa”, unless you really are.
• You can’t call yourself “The Unnamed Person Mentioned in News Story X”, unless you really are that person.
• You can’t say “I have 13 PhDs in X, Y, and Z, and blah blah blah experience”, unless you do.

If you “clearly state” that you’re not person X, then you are not pretending to be person X.

I don’t know how you could actually be somebody you’re not.

Is it okay to pretend to be somebody you’re not, after one clearly states that they are pretending to be somebody that they’re not. And what not pretending, but actually being somebody you’re not.
Can any of this be a privacy issue?
With a pretense that does not result in harm, perhaps it is just a given , don’t pretend to be somebody that is hurting.