The reason the Alto included a GUI at all was specifically Alan Kay's vision, as explained in the text I quoted by the people who built it. That text includes primary sources written at the time by, among other people, Cedar's main author and Bravo's main author — not only, as you imply, stories written down from memory twenty years later. It also includes primary sources from many years later, but those add to the evidence; they do not subtract from it.

I do not believe that Wirth's "OO" is a different topic. To the best of my understanding, the reason he added procedure-typed record fields in Modula-2 and retained them in Oberon was specifically in order to implement that tiling window manager, which in later versions of Mesa (and Cedar) replaced the overlapping-window GUI in earlier versions of Mesa, which came from Alan Kay's 01969 doctoral dissertation, by way of Smalltalk.