You haven’t detailed how any of this is connected, and there aren’t Sonos 1’s, just Sonos Ones or PLAY:1’s, so it’s hard to confirm one way or the other.
I can think of two ways this ‘might’ work. If this Apple Airport Express device has a stereo audio output that you have connected to a Sonos line in, then all of the AirPlay 2 stuff is happening before the signal gets in to the Sonos ecosystem. Consequently, Sonos doesn’t care how the signal gets to the line in.
Alternately, you might have Sonos Ones, which are capable of receiving an AirPlay 2 signal. In which case you could group any other Sonos “room” with the “room” that is the pair of Sonos Ones, and play the AiPlay 2 signal anywhere in your system.
Or (if you have the know-how) just run AirConnect somewhere. Instant AirPlay (v1) support for all of your Sonos devices.
https://github.com/philippe44/AirConnect