I don't think this is true, in any case you can extrapolate, e.g. if you're leaving from gate 34, see which flight of this airline is arriving at gate 34 just before, then you can check that flight's details, including plane model, on the tracking websites
My kid is a huge fan of flightradar so we spend way too much time watching it. I don't know the exact timing, but it is a consistent behavior that planes pop onto flightradar only a little bit before pulling out of the gate. But I'm not sure if there's a standardized point in the sequence when they turn it on or it's up to each pilot.
But you're right, watching the incoming plane to that gate would be a good way to know earlier!
