Hey, I’m the main researcher working on this. Good questions. The words “secure” and “security” are very special words with deep meanings to me. I use them very carefully because they compose many things. Unless my Ctrl+F is failing me, I don’t see them use in this article and they’re not used in the original paper either. I see the term is used in the blog post (emphasis mine):
Leveraging blockchain technology allows us to do two things that are integral to the success of the framework: offer participants a level of trust and security and reliably execute an incentive-based system to encourage participants to contribute data that will help improve a model’s performance.
Here the security is about transparency. Open-source code is great but it’s not sufficient. For example, even if the back end code for this website was open-source, I don’t know what really happens when I enter my comment into this box because I can’t verify that the code running on back end machines is the same as what is in the open-source code that I read. Smart contracts offer a stronger guarantee that the code we see is the code that runs.