Solid question; there are only third-party apps. A recent discussion in !syncthing@lemmy.ml led me to most recently adopt BasicSync, which is incredibly low-profile and is probably the closest thing we can get to it.

However... if you want to get as pure as possible, you can apparently run Syncthing's Linux version directly in Termux on Android without the need for a dedicated Android app. There are also entire alternatives to Syncthing like syncspirit (which can also be run through Termux and which I'm considering trying as well).