No depressurization, intercept pilots were able to see the pilot slumped over, windows would have been frosted over if a decompress, most likely a medical issue.
Failure to pressurize isn't a depressurization. Humidity vents overboard along with the cabin air in the climb. The pilot -- if he's still breathing -- still would be respiring moisture.
OpSpec SOP of many organizations includes turning on windscreen anti-ice in the run-up in the belief it will make the windscreen more pliable and less likely to shatter in the event of a bird strike. Low humidity and operating windscreen anti-ice would have reduced and could have eliminated the tendency to ice. I think it's speculative to say the icing had to have been so severe that the intercept pilots could not possibly have seen the outline of a shape slumped forward in the left seat.
I'm not arguing it wasn't a medical, I'm arguing that isn't the only option that fits the known details.