No major company that I am aware of fills their IGU's with argon to a pressure greater or less than the pressure at the location where the IGU's are filled.
Marvin IGU's are manufactured by Cardinal and they are filled and sealed in a vacuum chamber.
Cardinal (per their website and literature) has tested their argon filled IGU construction in units that pass over the mountains and they warrant those units against any sort of damage caused by the change in atmospheric pressure during the trip.
But, at the same time, they do not recommend that these units remain at altitudes 5000' above the altitude where the IGU's were manufactured.
The pressure in the IGU that much above the manufacturing altitude will cause the windows to bow out significantly and will cause a very noticeable visual distortion.
Breather tubes and capillary tubes are not exactly the same thing.
A breather tube is short, has a relatively large opening, and is aluminum.
A capillary tube is long, has a very narrow opening, and stainless steel.
Both devices are designed to equalize pressure within an IGU at high altitudes, but there are a few differences.
Breather tubes are desgined to be crimped closed when the window is installed.
Capillary tubes remain open to allow for barometric pressure changes. Because of the nature of the capillary tube and because the air at higher altitudes tends to be very dry, moisture isues within the IGU is not considered to be an issue, despite the open tube.
Breather tubes tend to allow significantly more moisture into the IGU en route to the eventual installation site than do capillary tubes due to the basic design of each device.
There is a very common notion that argon gas disipates out of an IGU in 5 to 7 years. This was common at one time. It is no longer true of the top manufacturers using the newer spacer systems, but many salesfolks, and some knowledgeable folks will still tell prospective buyers that it is true.
Long term weathering and accelerated laboratory testing of several of the newer spacer / IGU systems is pretty conclusive that the better products will hold argon for the life of the window unit - 50 years or more.
Actually, several IGU / spacer manufacturers (particularly EdgeTech and Cardinal) are pushing for the hand-held testers to have much more common usage in the marketplace in order to prove that their products do actually perform as advertised.