True, but also irrelevant. The real question is whether Kobo is making enough money from XP users to justify supporting XP. It is a question that only Kobo can answer, since they will have (or at least should) have a record of their sales breakdown according to platform. That breakdown would include things such as sales via a browser vs. sales via Kobo Desktop, and likely includes the usage of Kobo Desktop for reading. (Keep in mind, XP users can still make purchases via the browser or ereader.)

As outsiders, we can only make guesses at these things. We can look at general data, which shows that XP's installed user base exceeds that of OS X, where the former is unsupported and the latter is supported. Yet we don't know how that translates into their customer base (e.g. OS X users may be making more purchases or purchasing more profitable titles).