Previous model was usual open source community-driven project with donations and it's obviously didn't worked, but Synergy was developed by different team at different times and looks like they find what works for them. It's tarted 3 years ago:

https://www.reddit.com/r/linux/comments/2fydpm/synergy_gpl_i...

Of course it's not shiny VC funded company with 10000% growth, but I guess it's profitable business. Though it's not that much different from what Red Hat doing by selling support even if it's on small scale.