This check should indeed be done. Just curious: Why is the new Seafile Client version no longer compatible with OSX El Capitan @daniel.pan ?
Here are at least links for a downgrade. It seems to be enough to change the version number to load the desired version:
https://s3.eu-central-1.amazonaws.com/download.seadrive.org/seafile-client-7.0.4.dmg
Update: The changelog indeed mentions drop of support for OSX versions below 10.12: https://download.seafile.com/published/seafile-manual/changelog/client-changelog.md
Unfortunately 7.0.4 seems to be the smallest version on AWS storage but 7.0.3 would be needed.