I have no clue what happens with permissions on windows. On our side we tell the golang standard library to chmod to whatever permission was set on the linux side. As there’s probably no direct windows equivalent for that (otherwise you probably wouldn’t need to configure fallback perms for WSL), they are probably “converted” to include execute perms along the way.
If you care about linux perms in WSL (aka work in there), I’d guess that’s a good idea. It’s possible that there’s limitations in WSL that affects Syncthing (e.g. filesystem watching), but I wouldn’t know.