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It's is like saying "only blonde people should be in this room". If there's nobody in the room, it still holds.

Replace room with [], and "blonde people" with "X matching C condition" and you get:

"only X matching C condition should be in this set"

which expresses all([]).

It only looks strange for people who somehow misunderstand all([]) to mean:

"there must be at least one X in the set" && "only X matching C condition should be in the set".

P.S. Nominally all(...) just checks for a single condition: X is True, but it's trivial to get a list of True/False values from any list of other values via mapping it through a boolean function.