The SyncThing developers wrote on their website that they don't offer an iOS client because Apple doesn't properly support background processing. It seems that the SyncThing developers don't care about iOS.
But here's the beauty of Open Source: anyone can download the source code and try to get it running on iOS.
Now Möbius found a way to get SyncThing kinda working on iOS, and for a small fee you can install it on your iPhone. The wrapper they built around SyncThing isn't open source, but for Möbius customers that doesn't really matter. They just want something that works. And through the beauty of Open Source, someone was able to build it for them, and they get to make a little money with it!
Yes, many proprietary software developers who don't respect the freedoms of users make money piggybacking (some might say parasitically) on open source software releases.
I'm not sure what this has to do with open source, however. These people aren't even part of the free software community, as they obviously don't care about software freedom.
It sounds like you are confusing open source with copyleft.
Copyleft is a great thing, but it's also fine for things to be open source and not copyleft.
I can use syncthing because of Mobius. If Mobius did not exist, I would either have to pay a bounty to get someone to port Syncthing or use something else. That bounty is more than $6.
You can still port syncthing yourself. Mobius is not stopping you! You'll have to pay Apple $99 a year, and you will have to handle the app store process, and all - but you can do it. Neither syncthing nor Mobius will stop you.
Joining the Apple developer program requires that you dox yourself to Apple, and ties that government identity to device hardware serial numbers. I buy my Apple devices for cash and use each one with a unique disposable Apple ID created with a burner email address and burner prepaid phone number.
This is also one of the reasons I can't pay for IAP cancer - attaching payment methods links identity to the Apple ID and hardware serial number.
But here's the beauty of Open Source: anyone can download the source code and try to get it running on iOS.
Now Möbius found a way to get SyncThing kinda working on iOS, and for a small fee you can install it on your iPhone. The wrapper they built around SyncThing isn't open source, but for Möbius customers that doesn't really matter. They just want something that works. And through the beauty of Open Source, someone was able to build it for them, and they get to make a little money with it!