Irrelevant (because the assignment of twins to particular environments is mostly blinded to mutation differences) and pedantic (because the differences are trivial).

Wrong, and wrong. Keep pace with the literature. Differences can manifest as early as embryogenesis and have far reaching outcomes, e.g. there have been reports of different covid severity outcomes within a twin pair.

Hell, you do know that the cells in your own body don't all have the same genome, right?

As I said, it's irrelevant since assignment is mostly random at birth. The law of large numbers takes care of trivial differences since those differences are largely randomized.