It seems this feature was left due to limitations, and ExtJS seems to suggest to disable this feature completely currently.
There's a related sencha issue that at least improves zoom handling and that has been fixed on upcoming ExtJS 6.1.0: ExtJS 5 zoom and scroll on touch devices
This was reported in at least two Ext.NET forum threads:
Better test it after examples explorer is published. According to the user (see 'Mobile' forum thread above), ExtJS 6.2.1 addressed the issue.
The example provided by the user in thread Pinch to zoom mobile devices doesn't work worked with pinch zoom under google chrome emulating both apple and android devices.
Feedback provided to both related forum threads.