As this isn’t an official dictionary term, we can’t just have one official definition, so I went to Google and picked the first 3 results for “what is pay2win”. Here are the results:
“Games that let you buy better gear or allow you to make better items then everyone else at a faster rate and then makes the game largely unbalanced even for people who have skill in the game without paying.” (Urbandictionary dot com)
"Pay-to-win, abbreviated as P2W, describes an aspect of a game where players are allowed to get an advantage with real-world currency to purchase in-game items. The pay-to-win structure is prevalent in both multiplayer online games through DLC (downloadable content) and mobile games through in-app purchases.
Pay-to-win games usually offer better weapons, armor, character abilities, or in-game currency to players as microtransactions. The pay-to-win advantages make it harder for other players to compete if they cannot afford or do not want to purchase the same items. As the definition would imply, players essentially pay to win at a game." (Computerhope dot com)
"In the simplest terms, “pay-to-win” describes a game where paying real money gives players a significant advantage over those who don’t spend any money.
Simply having microtransactions does not make a game pay-to-win. Most games these days, especially multiplayer titles, have microtransactions, loot boxes, battles passes, and similar monetization systems." (Makeuseof dot com)
As you can see in these 3 definitions, it isn’t enough for the game to offer ways to obtain something through real money. The advantages you get need to be significant enough (even unbalanced) for the game to be considered pay 2 win.
In WoW, whether or not you can complete high difficulty content depends on your skill. You can do it a little faster by spending money, if anything, but that’s it.