Bitcoin is the chain originating from the genesis block with the highest accumulated proof of work. The Bitcoin Cash fork failed to gain majority, thus it is not Bitcoin.
In practice this is actually a fairly shortsighted way of looking at it, because accumulated proof of work only becomes the deciding factor once other necessary attributes have been confirmed.
(This btw is the only good reason underlying why so many have been sucked into the populist notion that users voting with so called "full nodes" - that don't fulfill the criteria for being called nodes as laid out in the design paper - can be an acceptable governance mechanism, when it is in fact much too easily manipulated.)