Yes, determining life and death should be considering hypothetical ideal lines of play, so it’s like using a copy of the board, while leaving the final board position untouched. There also other complexities when figuring out the life and death status under Japanese rules, such as applying a different ko rule that essentially nullifies ko threats.
I made an entire thread about Odd Cases 🤔 in the Japanese Rules
For beginners, it’s a reasonable mistake to make, but playing out life/death disputes under Japanese rules can lead to incorrect outcomes, which is not ideal.