WikiTree has no official definition of "ancestor", so the term is used on WikiTree however the multitude of individual WT members use it, which may vary by the context.
In the English-speaking world, the term is most commonly used with the narrow meaning of a person from whom you are descended. However, it is not uncommon (and therefore not wrong) for it to be sometimes used with a broader meaning to include other long-ago relatives, such as great aunts and uncles. For example, if someone is asked if they have any famous ancestors, they might respond by mentioning a famous great uncle. Some sources specifically condone such a broader meaning. For example, here's the definition from MyHeritage (bolding added):
Ancestors
Your ancestors are the people who came before you in your family. The term usually refers to relatives more distant than grandparents, such as your great-grandparents, great-great-grandparents, and so on. Ancestors may also include people from whom you are not directly descended, such as great-aunts and uncles.
And the Cambridge Dictionary defines "Ancestor" as:
a person related to you who lived a long time ago.
Which could include, say, great uncles and aunts.
The possibility that some people might interpret "ancestors" to include people other than people from whom you are descended is no doubt the reason that lots of people use the term "direct ancestors" when they want to be clear that they only mean people from whom they are descended.