Welcome to the forum. :) 

You are correct in your understanding that Play:1 devices are not compatible with the Arc Ultra as surrounds.  I suspect that this is due to hardware used.  The App compatibility is normally driven by the amount of RAM and processor power, as such I would consider it virtually impossible for Sonos to ever support them as surrounds for the Arc Ultra.

Edited: To correct mistake about App version.

I suspect that this is due to hardware used, as the Play:1 is only supported on the S1 App, whilst the Arc Ultra is on supported on the S2 App. 

The Play 1 IS supported on the S2 app. I got an Arc for Christmas last year and had to “upgrade” to the S2 app. My old Sub and Play 1s were still able to pair with the Arc just fine.

Its is more of a hardware compatibility issue as opposed to software compatibility.

play:1 is S2 compatible so it’s not for that reason. It still might be the hardware or maybe they want to push owners to new Era speakers, who knows. 

Sorry, well spotted: I misread the article:

https://support.sonos.com/en-gb/article/sonos-app-version-compatibility

Thanks All for the responses. Greatly appreciated 

You could try and ad them - on the Dutch language forum someone posted he succeeded in adding his Play 1's to the Arc Ultra: Play:1 speakers als surround speakers en sub bij Arc Ultra | Sonos Community

Thanks.  Interesting if not a little confusing…….I wonder if an App update may nullify that configuration in the future?

Another user also had success here:

But future app update will most probably stop any new bondings with Play:1s.

They will force us to update the play1.... Money money monkeys

I love that ABBA song.

Lol

Given Play:1s have been successfully added as surrounds by some users to Arc Ultra, the intended limitation is clearly not hardware related.