I'm not sure which examples in the notes on line you are referring to, but the examples that I see are copular constructions, where the copula is usually elided.
. Is dochtúir é = He is a doctor = "[It is] a doctor that is in him" = (Is) dochtúir atá ann
. Is feirmeoirí muid = We are farmers = "[It is] farmers that are in us" = (Is) feirmeoirí atá ionainn
. Is dlíodóir í = She is a lawyer = "[It is] a lawyer that is in her" = (Is) dlíodóir atá inti
You can't use the copula to say "only", so the fact that the structure using níl ach isn't similar to a copular sentence isn't surprising, but on closer inspection you'll see that the preposition is the "subject" of tá in each of those examples (atá ann, atá ionainn, atá inti), just as it is the "subject" of níl in Níl ionat ach innealtóir
(The preposition isn't actually the subject in these cases, it just comes immediately after the verb, where the subject normally goes).