Perhaps it is a much younger Arthur Langtrey (say 200 years younger) and the Person we have been assuming is The Merlin, isn’t. It’s Cowl. Cowl was the former Warden, somehow corrupted by tapping into the power of Demonreach (Harry has been warned), and eventually divorced from Alfred because of it.
He deserves to be there because he was tricked into it, and may not realise Harry is a different Warden. Through in a Body swap and entirely possible, Cowl is keeping his original younger body as a backup.
It would be very Jim to say he isn’t The Merlin, when in fact it is a Merlin.
Of course Turncoat needs explaining under this scenario.