Then clearly fighters can't be Rune Knights, since that is optional and in Tashas. They may be able to subject to their individual DM.
But your original post talks about RK's as if they are fighters in 5E .... just like my post talks about fighters switching subclasses in 5E as if that is something they can do in 5E.
Can fighters be Rune Knights in 5E?
Can fighters switch subclasses in 5E?
Do you not see this is the same thing, using an optional rule? Do you not see the double standard here?
It's not a double standard. I'm not claiming anywhere the Rube Knight is actually available it's up to the DM.
It's two different things. A player might be able to play a Rune Knight a Fighter may be able to change subclass.
Player, PC two different things. One or the other, both or none may be available.