How much current would flow if 2 x 18650 batteries are shorted due to fault?

It also depends on the battery. When I short my 18650 LiPo batteries (I have accidentally done this), nothing spectacular happens. The battery's built-in protection circuit shuts it down (no voltage produced any more!) until I place it in the charger, which resets the circuit. I've never used them for more than about 2A of current - powering two small DC motors so no idea where the limit is before the protection kicks in.

For unprotected batteries, yes the current can be huge.

Note that the specs say "below 50 mΩ" for the internal resistance. As that's apparently a guaranteed value, the actual value could be as low as 10-20 mΩ. LiPo battery applications include RC boats and planes with high-powered motors that draw tens of amps continuously.