The threat model described by the post above you is actually not about physical access. It's about the PI getting hacked remotely.
If you use Syncthing's encryption then at no point is the decrypted content available to the PI. It gets decrypted locally by other Syncthing peers after they have downloaded it.
Besides, there's still a difference between physical accesses: plain and non-targeted (besides how profitable they're expected) burglaries are way more common than violent targeted attacks meant to extract a secret from an individual.
If you use Syncthing's encryption then at no point is the decrypted content available to the PI. It gets decrypted locally by other Syncthing peers after they have downloaded it.