I'm experiencing delays like this for my macOS apps, once for a new app, and another for another app's update, however I never got a message from Apple by themselves, but when I emailed them, I got a similar response that you got.
The new app was submitted on August 30 for macOS after being rejected twice, was "In Review" that evening, then was accepted on September 7.
The update for another app I made was submitted for iOS & macOS on September 22, the iOS version was accepted on the 23rd, and the macOS version has been "In Review" for 2 days so far and counting.
I guess the only thing we can do is wait.