It wasn't an emotional letdown game as much as a preseason game. By that point the Bills were essentially locked in at #2 seed, and were NOT going to open the playbook and lay down film for playoff opponents. Hardly anyone was getting open anywhere, Brady didn't really run anything to attack the Pats that wasn't already on film already. The previous month they'd be going all-out toward first the division title (SF game) and #1 seed. After the Rams loss and Lions win and KC continuing to somehow not lose, the final 3 games became irrelevant - and the Bills still won till they fully emptied the bench Week 18.
Lamar's passing EPA was better which is the only stat that's important for QB's unless you are a running QB in which case the combined EPA is very important unless unless you're Lamar Jackson and your running EPA isn't very good in which case it's not important at all.