Thermal noise voltage measured by an impedance matched ADC
Is this a correct conclusion?


No, for two opposing reasons.

In theory, an amplifier can have a noise figure of less than 3dB, because it is not in thermal equilibrium, and the relationship between input impedance and noise only holds for a system in thermal equilibrium.  So if you could build an ADC that acted like an amplifier, you could get a very low noise figure.

But, life is not so kind.  In practice, ADCs have lots of excess noise.  I'm not enough of an expert to tell you why -- but it is, it has been, and it probably will be, even when we're fending off Romulans and Klingons.

I'm sure that part of it is because the comparator needs to be very wide band compared to the sampling rate, and because sampling aliases the noise into baseband.  I'm sure that another part of it is because of necessity ADCs are mixed-signal devices, so a monolithic ADC needs to use a process that is a compromise between good digital performance and good analog performance.  Yet another part of it is because digital circuits are noisy, and an ADC puts a sensitive analog circuit right next to a digital one.