Interestingly enough, trackpad pinch-to-zoom works in Acrobat XI on the same computer! So this does appear to be a bug, at least on Mac OS 10.9.5.
Also, in the browser plugin of the Acrobat DC, pinch-to-zoom kind of works, but behaves in a weird way. After a couple of in and out pinch-to-zooms the rendering quality of the PDF deteriorates to non-retina quality. This might be related to another long-standing bug that in full-screen mode only the first page is displayed in retina quality and switches to non-retina after that.