Why, because the default Bitcoin software checks the genesis block. I would call this a checkpoint no matter what Wikipedia says and what unintuitive definition of checkpoint they use. I see no reason to exclude a fixed beginning from being a checkpoint from an abstract point of view.
The genesis block is part of bitcoin. It is not a checkpoint according to the generally-accepted definition of the word "checkpoint" as applied to computing. A checkpoint is a point
beyond the beginning.
If you make up your own definitions for words, however, you can argue anything you like.