Hello,
pinch zooming worked in ELAN versions that were distributed with Apple’s own Java implementation. This didn’t work anymore after upgrading to newer OpenJDK Java versions (I will have to check whether the latest OpenJDK implementations maybe re-introduced support for this type of gestures).
If you want to zoom with the Trackpad, you can hold down the Control key and scroll-up or -down with two fingers. The keyboard combinations Command +, Command - and Command 0 should also work.
More on the zoom slider can be found in this post.