If you like to speculate, I saw the following in a preview video of the adaptive interface:
I don’t know what happens if you click on nextcloud there but it looks like gnome has a Nextcloud integration already in?
However I see it as Caliga - they will probably announce it as a feature with the PureOS Store or maybe once librem one goes to Librem Files (or Photos).
However there was also a discussion about having the librem 5 as a “cloud phone”, which would literally mean all your data is stored elsewhere (e.g. in your own nextcloud - as most of us probably do with caldav, carddav and photos via nc)