Widgets with other data types on the x-axis do not get synchronized.
The synchronization is also applied to widgets without horizontal zooming enabled, as long as they satisfy the above conditions. But you will need at least one widget with zooming enabled.
If a section has synchronized zooming all widgets with timestamps on the x-axis will initially display a time frame such that all data is visible.
You can activate the zoom synchronization in the settings of the dashboard edit mode. Just select the section in which you want to have synchronized zoom and click the check box.
Please note that, if you have charts which use a zoom navigator, it may be that after loading the dashboard, only the zoom navigators x axis gets synchronized but not the main charts x axis. This happens when a chart with a zoom navigator doesn't have data points for the whole time frame, and therefore as a default behaviour the zoom navigator zooms the main chart to only show available data.
You can see this effect in the image below where the second chart covers a bigger time frame than the first chart. What happens here is that the x axis of the second chart and the x axis of the zoom navigator get synchronized, but the chart itself on the first widget automatically gets zoomed in to only show available data.