The bottom end of the dipstick is below the surface of the oil in the crankcase. If enough crankcase pressure built up to force the dipstick out of it's tube, the exiting dipstick would be immediately followed by a column of engine lube oil.

FWIW, there's a tiny hole in the inboard side at the top of the dipstick tube that allows the pressure in the tube to equalize when you insert the dipstick. This ensures that you always get a correct oil level reading on the dipstick, not one that is artificially low because air presure in the dipstick tube temporarily forced the oil column downwards.

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