Dropbox has native applications for Debian based distros and Fedora. The Flatpak is an unofficial release by someone who has packaged the source for flatpak. It does look like it was updated 11 days ago so it might be worth reaching out.
Otherwise Dropbox does have instructions on how to build from source.
You can always submit for a package request here: Submit Package Request
Or if you are feeling froggy you can package Dropbox yourself for Solus 😃 Packaging Overview