Well yeah, strong relativism is a steaming pile of nonsense. But I'm not sure that you need strong relativism to get future-oriented behaviour differences, weak priming effects could well add up over time to produce noticeable differences in amount of savings over a long period.
What kind of factors did you have in mind? As far as I know they controlled for country, city, religion, income, family values.. probably some other stuff I can't remember. And at least some of the pairs of households he compared were in non-Western countries, again I can't remember off the top of my head.
I agree that less-than-strong forms of relativism can have an effect; priming effects are well enough known that it would be downright weird if human language was somehow exempt. But, again, this comes down to the non-linguistic factors, and distinguishing their effects from the effects of linguistic priming.
I can't address the specifics of how well they conducted their study - it's something of a black box to me. However, the graph around 8:15 in the video shows some seriously wonky stuff that I will flat-out refuse to accept as properly controlled for me...
http://www.youtube.com/watch?v=CiobJhogNnA
The short version is that if the language you speak requires different verbs for the present and the future, it causes you to think about it differently. Depending on the magnitude of the effect, this has important implications for construal level theory. If your language allows you to think about the future in Near mode, it may allow you to think about it more rationally.
Previous discussion on one of Keith Chen's papers here.