Earlier quoted context omitted.
A "summary of all human knowledge" that deliberately follows up with "well, actually not all human knowledge" is either dishonest or a failure.
They are different statements. They just sound similar. "The sum of all human knowledge" would suggest that everything in Wikipedia is known to be true, and everything known to be true is in Wikipedia. In "a summary of all human knowledge," the indefinite article and the word summary both imply a point of view. The sentence says that the knowledge has been abridged, and therefore, that there exists an abridger. It sa…
An abridged summary that editorially chooses which knowledge is worthy of summarizing is another thing.
Wikipedia is the latter. The "all knowledge" bit should not be part of the description, because it's not true. Wikipedia actually attempts to create "a summary of selected portions of human knowledge, chosen by those users with the most willingness to endlessly argue on the internet about what should and should not be included".