Looking at that page, it looks like syncthing chooses to not provide a Windows service. That is where 100% of the complexity is coming from. If they did provide a Windows service, then it would be as simple as opening the service manager, and setting the startup type from "manual" to "automatic".
It's not fair to blame Windows for the developers of an app not using its features. Similarly, if syncthing didn't bother to create a unit file on Linux, your example would no longer be a simple one liner. That wouldn't be systemd's fault though.