It seems to me there are two dynamics (perhaps more) at work. There is the issue of Oxygen change and I agree that it would call for a leaner mixture as is typical to many carbs. However, the Walbro uses a metering system on the float needle that is unlike float bowl carbs in that it uses atmospheric pressure as the constant in providing flow/pressure metering ahead of the adjustment needles.

When examining the Walbro, we see that greater atmospheric pressure leverages the float needle open for greater flow/pressure. Reducing atmospheres then lessens the flow/pressure requiring opening or richening the needles. The question is which dynamic has the greater affect and nets a change in which direction?