I find it interesting that Yuji managed to expand his domain without needing to mention her name.

After all, even people with 0% knowledge of Domains could sing and perform the same thing.

And the only factors that could have done it would be curses like Smallpox or the finger curse, However, they are curses, While the narrator specifies that it only applies to "Sorcerers," I don't think curses are imposed as a rule in the same sense as for sorcerers.

And also, Gege was practically still creating rules about jujutsu, so there are many things that are really different, and soon after he created something like this.