Van, there is a bit of a wive's tail where silicone fluid and altitude are concerned. It was never actually "high" altitude that was the issue but, instead, too sudden changes in altitude and, more specifically, the changes in barometric pressure that go along with it.
The theory goes like this. It definitely is true that DOT5 entrains air. It does. No doubt about it. So even a well bled system is going to carry itty bitty tiny bubbles of air within the liquid. That's ok actually. The problem is only when they are too many or too large.
The problem, however, is supposed to occur if, for example, you bled your brakes at a low altitude. Then you climbed. Lower pressure as you gain altitude is less pressure on the air bubbles themselves. This allows the air bubbles to expand. So these suddenly bigger air bubbles inside the system make for a spongy pedal.
Trouble is that once sealed the hydraulic system basically "protects" the enclosed fluids from the external differences. If it rains outside the fluid doesn't get wet. And if barometric pressure outside goes marginally up or down you can still drive over the mountains and not feel it in your brake pedal. The biggest thing with DOT5, no matter the altitude or other circumstances, is to get it bled slowly, carefully and well the first time. Once you get a a solid pedal you'll keep it (on a non-ABS vehicle).