The Angular Report Viewer uses the jQuery npm package, while the Web Report Designer uses jQuery.min.js directly from a CDN. If you try to use both Angular Viewer and JS Web Designer in the same application there will be a conflict while loading jQuery twice from the different sources. This is why the Angular Report Viewer cannot be loaded together with the JS Web Report Designer in the same application. The KB article Displaying the Web Report Designer in Angular application provides a Known Issues section at the bottom where this problem is explained:

"Telerik Web Report Designer could not be integrated into Angular application together with Angular Telerik Report Viewer. That is why this article shows how to use the Telerik Web Report Designer in angular application with jQuery-based HTML5 Telerik Report Viewer."