Other big confounding variable: the "experiment" consists mainly of asking people to recall or imagine a times when someone declines to attend something due to lack of time. Most people's life experience is going to include a vast number of invitations declined by a vast number of friends who were doing something else at the time. The experience that springs to mind
first is probably an unusually disappointing one.
(cf invitations declined due to cost, which is somewhat rarer. First example that springs to mind is probably something that's very expensive, or someone that is particularly poor or would have a particularly expensive trip to meet up)
I don't think that translates to real world acceptance of financial excuses generally being higher, especially not if you're on a software developer's salary and the invitation is for coffee...