Please correct me if I am wrong, but pinch zoom is currently only implemented for SAFARI (and there it's implemented using the non-standard "gesture" API, in the EditorUI.js example)
https://github.com/jgraph/mxgraph/blob/ff141aab158417bd866e2dfebd06c61d40773cd2/javascript/src/js/util/mxEvent.js#L153

Here is the example of "touch" support from the docs:
https://jgraph.github.io/mxgraph/javascript/examples/touch.html

source code:
https://github.com/jgraph/mxgraph/blob/master/javascript/examples/touch.html

This example makes me really sad.
Would be great to have the pinch support implemented properly.