The question is mostly answerable. It's actually quite easy to validate the null hypothesis Moldvay Basic is the primary source of the term XP.
All one has to do is show that either Moldvay wasn't the most widespread early source, or that a similarly widespread source was earlier. In point of fact, both of which are true. (AD&D's DMG uses XP, is earlier, in print longer, sold more copies, and in general, was better known prior to the rise of the OSR.)
History of Gaming, especially history of specific terms, is of use in later game design, as well as specific non-gaming uses (to wit, authors of fiction set in period).
Which said, this particular question isn't really off topic. Further, it's actually answerable with a relatively high degree of certainty - far more than, say, Encouraging players to not cheat the item purchase system?.
Plus, such questions are exactly the kind of thing best handled by crowdsourcing answers... which is exactly the purpose of SE sites.