I think he's saying that he's used to each program running in its own address space with its own windows all of which appear on a common desktop. Like a Mac or Windows machine today.

While in Smalltalk, Interlisp and Cedar, the language environment controls the entire machine with a common address space so every window is a view into code running (multithreaded) in a single environment. E.g. if you change a smalltalk method every window will immediately start using the updated code.