It’s about what you’d expect from the equations.
...although I believe there was a mistake. I think the starting pressure was 13.3 PSI, not 13.5.
“Thermodynamically severe”? That might apply if the rate of cooling were at issue.
Why didn’t the Colts balls cool as expected if the conditions wee identical? The most obvious reasons are they might not have been subject to identical conditions or testing. They certainly should have cooled at least from the max stated in the guidelines to below the minimum if they were under the conditions people generally think they were, and measured at the times people think they were.
The simplest explanation is that the assumptions there are flawed...as opposed to assuming that the Patriots cheated by NOT having magic balls that the Colts must have had.
You made it an issue by your statement: "Pressure drops from 13.5 to 11.9 in minutes".
The connection between temperature and pressure is well understood.
If the experiment was, as I suspect, an effort to demonstrate that the environmental conditions at game time were sufficient to explain the NFL's observation that the Patriots' balls were underinflated, the experiment was essentially meaningless.
We know the weather conditions, so we know the end point temperature, but we really don't know the initial temperature of the balls when their pressures were tested prior to the game.
Hopefully, the NFL has that information.