It is fully opensource, the opensource code contains also the logic to verify a serial key that you will be given if you purchase the license and if verification succeeds the code will enable the extra features.
As such you could fork it, tweak the code around the license verification to always return TRUE or whatever, and you would be running a "pro version" of the software.
The developer simply trusts that people who like the software will purchase a license, or the fact that the majority of people out there are not programmers and would not be able to rebuild the software for themselves. Besides, the product is also offered as a SaaS.
for those looking for an open source way to sync photos, syncthing has an android app that works well. While it is 'always running' on my android phone, I love how as soon as I arrive home it connects to wifi and moves any new photos to my home linux box (which is also running syncthing).
https://github.com/photoprism/photoprism/blob/develop/LICENS...
https://opensource.org/license/agpl-v3/