The factory hydraulic tappet doesn't pressurize the pushrod (as the V8 etc. does). There is no oil pressure outlet at the pushrod ball seat.
The tappet is designed to self-purge of air bubbles by having the highest point of internal oil pressure at the point where the plunger is secured by the snap ring. The air is continuously pumped out by fresh oil pressure from the galley.
I would guess the slots direct drain oil to the bottom of the cup to improve the ball's service life.
