It is common online to use the words "vegetarian" and "vegan" interchangeably. When a distinction is drawn, it is almost always over milk, eggs, and honey. In the context of this article, those three items are never mentioned. The author is using "vegetarian" loosely to mean "eats vegetables," not strictly to mean "includes milk, eggs, and honey." This word usage produces some ambiguity, but it is common.
So, that doesn't mean that the source is "wrong." The claim about shao-lin monks eating a vegan diet is still true, and confirmed by other sources like this one (and others if you care to search yourself). It's all vegetables all the way down.
The "eight treasures congee" is also a dish that has cultural variants. So, more ambiguity. However, it is USUALLY fully vegetarian. Some people add meat for various reasons, but this ingredient is not a core part of it in most versions, and is not present in the version used by the shao-lin monks.
And I seriously doubt that many monks are vegetarians
Well your intuitions on the topic are not supported by the facts. There may be other monks that eat meat, but the Shao-Lin monks do not.
However with a good diet, you do not need meat, if that was your point?
Yes, that was my point. Shao-lin monks are just an extreme example since they exclude meat and are so athletic and healthy.