When control described it as "discrete" was it because they know that listeners wouldn't be prepared to monitor it? I find it odd that a public frequency would be described as discrete without some greater barrier to listening in.

You are thinking of "discreet":

* careful and circumspect in one's speech or actions, especially in order to avoid causing offense or to gain an advantage.

The word they were using was "discrete":

* individually separate and distinct.

120.15 is/was not used for anything else at Denver, so people could talk about the situation on the ground without effecting the operations elsewhere. Once the problem was 'contained', the rest of the airport could go back to operating 'normally' on the other runways.

I see. Thank you for clarifying that.

I'm still a bit surprised that the audio went uncaptured. It is interesting how much information that would be interesting to the public still spills into the ether.