All SnapEDA parts have slots represented on the Edge.Cuts layer.
To be able to find the real shapes of the slotted holes, please select View > Drawing Mode > Sketch Pads.

Technically, support for these cutouts outlines does not exist in KiCad but it works fine and board manufacturers will understand that this is a cutout in the gerber files. However, you should tell your board house that the information on the gerber file (containing the Edge.Cuts layer) needs to be milled from the board.

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