There's almost a reasonable point here among the ridiculous writing: philosophers become "great" not because they were necessarily right about everything, but because of their influence. That's not particularly in dispute; I don't think even the most ardent fans of these three philosophers think they were literally 100% correct or that every argument they made was equally unassailable. I don't think anyone thinks that about the intellectual giants in any field.

However, the examples Huemer picks on here aren't particularly good ones, and all of these deserve vastly more serious treatment than he gives them. Imagine deriding Kant's ethical theory, much less the entirety of Kant's work, based on one formulation of the categorical imperative and one quote from the Groundwork! Imagine critiquing Hume's "bundle of perceptions" theory of the self, without noting how it set the stage for Kant's unity of apperception.

That last example is significant, because where this article is absurdly incorrect is in further saying these were bad philosophers, bad thinkers, or that their "greatness" is entirely due to the provocative nature of their ideas. These people are great because of the groundwork they laid for the further development of philosophy, not because "whoa, dude".