Indeed we renamed "Workspace" to "App" about a month ago. We did that gradually by first running the idea through our customers, letting some of them try it out and then we released it officially. We used the Telerik Platform notifications panel to notify our users for the change.
We also tried to update as much of our resources as possible, which is why you weren't able to find the any mentions of a 'Workspace' in the documentation. Still, since we have many resources out there, there still will be old videos or blogs which will refer to the Apps as 'workspaces'.
I do apologize for the confusion and I will gladly address any further questions or concerns you might have on the topic.