It doesn't allow to display a Cesium viewer in Observable. That would be way too complicated because Cesium loads a lot of data files. It should work without problem if you just want to use the mathematical helpers of Cesium.
Note that after the publication of this package I figured the team behind Cesium already provided an UMD version. It's not very well documented in their doc but it is used in some of their examples. So you probably would prefer to use it instead.
Usage in Observable
Cesium = require('cesium-umd@the-version') Cesium.Cartesian3