I would probably have expected this to be called "high reverse isolation amplifier".
"Reverse isolation" measurements will be at the nominal expected impedance though. A S12 measurement for gain and S21 for reverse isolation. I.e. A high reverse isolation amplifier What you use in a receiver on a mixer input, if you were trying to minimize LO leakage to the antenna. It definitely is providing isolation, but I don't think "Reverse isolation" is the right term, especially since is providing a constant impedance forward and reverse.
First, you have S12 and S21 mixed up. S21 is the gain, S12 is the reverse isolation.
This amplifier would ideally be a "Unilateral amplifer:"
https://en.wikipedia.org/wiki/Amplifier#Unilateral_or_bilateralAn amplifier whose output exhibits no feedback to its input side is described as 'unilateral'. The input impedance of a unilateral amplifier is independent of load, and output impedance is independent of signal source impedance.
In terms of S-parameters, a unilateral amplifier is one with S12=0. There is no path from the output back to the input. So it has infinite reverse isolation.
For the input to be sensitive to the output, changing the load will change the load reflection coefficient. If there is no path back to the input, this change cannot be seen on the input and the input impedance does not change.
Similarly, if the source reflection coefficient changes. S22 is measured by injecting power into the output port and measuring the reflection. If there is no path in the reverse direction back to the input port, then the a change of source impedance cannot be seen on the output.
It's all explained in detail in the classic app note from HP,
"S-Parameter Techniques for Faster, More Accurate Network Design":
http://www.hparchive.com/Application_Notes/HP-AN-95-1.pdfThe first equations on page 11 show that if S12 = 0, S11 and S22 do not change with load or source impedance.
Here's another good old HP app note "S-parameter design":
http://www.sss-mag.com/pdf/AN154.pdfSo a high reverse isolation amplifier is a "termination insensitive amplifier". (Of course the gain, output power, noise figure will all still be sensitive to termination. That's why I don't much like the name "termination insensitive".)