I surveyed this location today. Apart from the C3 sign, both information signs near the gate clearly state that cycling is not allowed, and there's an extra (small) C11 sign on the gate itself too. It is obvious that cycling is not allowed here.

I've retagged the `highway=track` to `highway=footway` as well (it fits way better), and made several additions and corrections around here in changeset 157594825.