I gave it a listen and I have to congratulate you: you really did a great job with your talk, and it must have been a success, or I don’t know a thing about LUG talks.
I can only think of one thing that you’ve let slip through the cracks: when talking about how the relay servers work and how they can help facilitate data transfer if all else fails, you do say that in any case it’s only the changed chunks of a file that are being sent through a relay server, so that should not be seen as a big deal privacy-wise, but what you forgot to mention (or I didn’t pay close enough attention when you did) was the fact that even those chunks that do go through relay servers are encrypted in the way that the relay server (or its admin) have no way of seeing what’s in them. So while a relay server admin can indeed have access to your device ID and public IP address, even they have no way of seeing even the tiniest portion of actual data you are syncing.
One thing I feel is worth explaining, for you seemed to struggle on this topic, is the way Syncthing goes about file ownership (user and group). As you correctly mentioned in your talk, those are not preserved, and the way it works (this is where you seemed to have difficulties) is Syncthing creates every file it needs to create owned by the user the Syncthing app itself is run under. So when you sync files from your main workstation to your laptop, those files get created on the laptop with your user as owner, because that’s the user that runs the app. If I add a file to a folder on my workstation (and the file is owned by me, naturally), and then the file is synced to my wife’s laptop, on her laptop it is owned by her, because her user account on that laptop is the account Syncthing app runs under. If you were to run Syncthing as root on one of your machines (just a thought experiment, not recommended, don’t do this at home), the the files created by Syncthing on that machine would be owned by root.
And finally, I can’t resist commenting on your Syncthing geography. Yes, the Syncthing Foundation that runs the project is a Swedish entity operating under Swedish laws, but that’s basically because the initial Syncthing author, our tireless and fearless leader Jacob the @calmh is Swedish. Other than that, the project is international to the highest extent. For instance, of the three Maintainers that currently keep the project running, Simon @imsodin is to the best of my knowledge German (or at least lives in Germany), Audrius @AudriusButkevicius doesn’t advertise his whereabouts but has a distinctly Lithuanian nickname, and Jacob is, as already stated, Swedish, so saying “the core team is Swedish” is somewhat of a misstatement (one could argue Jacob is the team behind Syncthing, and I know some would support that point of view , but…)
All in all: really, nice job, and I think I’m speaking for all the community when I say your time and effort you put into this are valuable and highly appreciated.