Well, the claim was "Graphical User Interface (GUI)", not "overlapping windows". There are many examples of GUIs without overlapping windows and even usability experts claiming that it makes no sense to overlap windows. If I recall correctly neither Mesa nor Cedar used them.
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In fact Smalltalk-76 didn't have overlapping Windows, and the people at Apple had already come up with that before seeing the Smalltalk demo. Bill Atkinson describes that here. https://www.cnet.com/news/tracing-the-origins-of-the-macinto...
The screenshot in the article (which _does_ have overlapping windows) is evidently Smalltalk-80 because Smalltalk-76 used the "hollow colon" character in its message syntax where Smalltalk-80 uses the typographic colon.
There might be a misconception. Have a look at Ingalls' 1978 paper "The Smalltalk-76 programming system design and implementation" (https://dl.acm.org/doi/10.1145/512760.512762) which clearly shows overlapping windows. There is even an online version based on the original source: https://www.lively-web.org/users/bert/Smalltalk-76.html