As near as I can figure, that's the whole point. You'll have a zero difference when you fire it up with everything at room temp....BUT as soon as the device warms, it'll start moving heat away at such a rate as to maintain the calculated (or nearly) temp diff above ambient.
If you stick your room temp amp into the freezer, same deal.....just that now your working against the freezers ability to pull heat out (pump it) 'uphill' into the room....creating the cold box. Amp doesn't know this and will still end up warm VS the internal cold box temp.....and the same # of degrees.
If you put a power transistor in an insulated space and power it up,
you'll have a meltdown / failure in no time. No place for the heat to go. somewhere above maybe 150 to 200c, it'd just cease to function. These limits are all straight physics and chemistry.
But, based on the math the device will shed heat at a certain rate. Once it's been on and is stable, it'll run pretty much a certain amount above ambient for a long time. Even in the freezer......
This is pretty clear from the link I posted.
Also, find a datasheet. Somewhere is the 'derate' for power devices...and maybe others. The derate deals with power and temp. And how much less the device will take and at what rate, as it warms.
The OP? Long gone, but still and all, I'd not run my gear in an 85f space. Any weak link will be ruthlessly exposed. Any dry heatsink compound which isn't doing its job....a nut/screw securing a power transistor has come loose. A dust bunny clogging heatsink fins or some venting,.... All can hurt your stuff.