I also can’t fault the dealer on this. Many repair issues on modern cars do not have clear-cut answers at best and some can be downright nightmarish to sort out at worst.
In fact, even some factory service manuals will state that if tests show Part X to be good no matter how many times it’s tested then to replace it and hope.
It also could be that a failed seat heater may have taken the module with it. That kind of thing is not uncommon at all with any make of car or system and could be thought of as a line of dominos with each one affecting the next in line.