Straight-forward: this question has caused an edit war and a close-reopen-close war. The argument for tagging and reopening is:

The splat book only exists in DnD-5e, so it's not guessing – Pureferret

Can/should this question be tagged as dnd-5e?

Important to mention something that Medix2 noticed: the original question does not include the full name of the book (Explorer's Guide to Wildemount). It only says "Wildemount".

Related:

• What qualifies as guessing on a system?
• When is it OK to assume the system without it being considered a guess?
• Does adding the system tag to this question violate the "never guess the system" policy?
• What to do when an edit guesses the system being used rather than waiting for the querent to clarify?

And many more.

Very important note

We already have the policy, which you can read here. This question is about the specific question linked and, even if agreed that that particular question can be re-opened, it should not, ever, be used as a precedent to open any other question without community consensus. If you want to change the general policy, do it in the policy discussion.

Full disclosure: the only reason I even opened this meta question was because the arguments, edit wars and vote close-open wars were getting out of hand in the main site and it felt like opening this question was the more appropriate place to discuss it.