This is a fair point, but more often than not people living paycheck to paycheck aren't as familiar with dollar cost averaging and are probably just buying in $20 amounts because that's how much cash they have at that time.
The OP also said it was based on 5+ years worth of data which probably means it is less about gas price fluctuations and more about buying habits.
It has a pretty graph at the top and the guy presumably really did crunch real data, so it gets upvoted. Also it's Brazilian, so it gives you a little taste of 21st-century BRIC economics. Hits a lot of buzzers.
The OP also said it was based on 5+ years worth of data which probably means it is less about gas price fluctuations and more about buying habits.