Your point about the semi-arbitrary nature of the poverty line is good, but the figure is already inflation-adjusted. There is a general confusion on HN when it comes to real vs. nominal statistics. [1]
I find it's generally safe to assume that most of the well-respected development organizations are made up of smart people that know what they are doing and will not overlook something so fundamental as inflation, even if they do use statistics to advance their agendas from time to time.