Since the amplifier has both voltage and current input noise sources, there is an optimum source impedance that provides a minimum noise figure, and it’s not an impedance match. This is called a minimum noise match, along with associated noise contours where noise is traded off with impedance match.

Also, the noise from an antenna is dependent on it’s efficiency and what it’s pointing at. Even if it’s input impedance is 50 Ohms, it can generate far less noise than the equivalent resistor.