I just find a question, Book comparing of Agile methodologies, where xp is synonym to windows-xp, which obviously doesn't make sense. XP can be used to describe Windows XP but it can also be used to describe Extreme programming. The mentioned question is off topic but it doesn't mean it should be automatically wrongly retagged.
I think that in most the cases, when somebody hears XP thinks to Windows XP, rather than to extreme programming.
The fact the question is using xp to mean extreme programming or Windows XP is not, I think, the reason the question has been voted to be closed, especially from who voted to close it as off-topic: The reason is that the question has been (probably) seen as a buying recommendation. The other problem I could see with the question is that there could be another book published right after the book somebody could suggest, and the answer would not be valid anymore after a new book has been published. (With this I mean that who suggested a book could change idea once a new book is published.) Also, every answer would be equally valid, and asking questions where every answer would be equally valid is something the FAQ suggest not to do.
As for which synonyms to use for a tag, that is something normally decided by the community; a moderator can directly create synonym tags, but the community can still express an opinion about the opportunity of using a synonym. You can report here when you think a synonym should not be used for a tag. The votes for your question give an indication of what other users think about using the synonym: Who down-votes your question is against removing the synonym; who up-votes your question agrees with you that the synonym should not be used for that tag, or not used at all.
The majority of people will think of Windows, when they see 'XP'. If this tag synonym were to be removed or changed to mean extreme programming, it would cause a lot of questions to be mis-tagged by people who followed their first instincts.
If a tag is needed for extreme programming, it should be something else.