Is Airport Extreme AC WiFi 6 routers Dual band

Shouldn't be an issue. It will still operate with the Wi-Fi 5 (802.11 ac) standard. Just not able to create a Wi-Fi 6 wireless network. You can imagine if, at every major Wi-Fi standard release, if all the previous release devices became obsolete, there would be a major pushback to implementing the new release. Right? That's why you still see Wi-Fi 3 (g) or Wi-Fi 4 (n) networks still running today.

The AirPort Extreme is a dual-band Wi-Fi router, but it is not Wi-Fi 6 capable. It was designed (& now retired) well before the Wi-Fi 6 standard became available.