If the account is intended to be anonymous, it should have its own keypair not shared with the real identity (or other independently anonymous account).

I say this regardless of whether public keys are being.. publicised. User database could be leaked, say, or public keys visible to employees/logged. OpenSSH literally refers to them as 'identities' - if you're trying to be anonymous/anon w.r.t. another it goes without saying that you need to not use the same identity!