PTC fuses limit current by increasing their resistance. They don't stop the current from flowing like a traditional fuse. Instead the current stabilizes at some high but hopefully not too destructively high amount. How high the steady current ends up being depends on the fuse's resistance, the nature of the fault attempting to draw too much current, and the capability of the power supply.
A much lower max resistance will mean the steady fault condition current will (probably) end up much higher. You could imagine the word "robust" to mean different things, but at the very least you shouldn't expect "robust protection" from a part which much lower max resistance.