One can embody a thing without fully understanding it. That’s a poor counter-argument.
The notion that Beatrice – and Meta-Battler’s – identity is uncertain is recurrent throughout the text. The big one comes in EP3 which is admittedly not referring to Yasu but it’s still relevant; Beatrice herself (that is, Meta-Beatrice) is subject to mood swings that seem to indicate uncertainty; there are multiple Beatrices at points; Battler doubts his own identity so much when Beatrice pulls the Asumu trick that he stops existing; Beatrice “dies,” is brought back by Battler, is different, then learns to become herself again; even Bern and Lambda and Erika get in on it.
It seems more probable that meta-world witches exist due at least in part to issues of uncertainty with their existence and reality which push them to do what they do: Battler and Beatrice are the elevated avatars of fiction who lack a direct line to their creators to nail down things their creators did not clarify; Bern and Lambda aren’t sure at times if they aren’t still entrapped; Erika isn’t sure why she exists or what she’s actually supposed to do; Featherine literally suffered from memory loss that changed her identity. Not wholly knowing yourself is fundamental to the nature of witches.
EDIT: It’s also worth pointing out that we’re told Beatrice’s legend is not wholly created by her original creator. It’s the nature of Forgeries, and if Yasu didn’t write Banquet or Alliance (which seems at least likely given the manga and general tonal changes), then Beatrice’s characterization is clearly being affected by something Yasu never had any access to. She is distinct from her creator in a rather obvious way.