Indeed, the Nextcloud snap bundles everything it needs, including PHP v7.2. v7.2 is still supported and receiving regular updates, and the snap maintainers (disclaimer: that's me) don't typically introduce changes without a reason. However, v7.2 will reach the end of active support in a month, which is probably why the app you mention notes that it will no longer support it soon. It will continue receiving security updates for a good while, but barring any issues, the snap will upgrade to v7.3 soon. You'll receive that update automatically.
In the future, you should directly notify the maintainers of issues like this, for example by logging an issue against the project. You can find the appropriate place by using
snap info <snap name>, e.g.:
$ snap info nextcloud