MYN23AE/A is this Code for Brand New Right?

Yes. The M at the front indicates Apple sold it as new.

(260227)

Stated another way, the "M" means that the iPhone was new when it was originally sold.

But, it does not tell you whether another user might have purchased the phone previously and returned it to the store and then the phone was repackaged and sold to you.

It is unlikely that something like this would happen with an authorized Apple dealer, but you would need to check with the store if you are still concerned about the condition of the phone.