On the downside, I looked at a few methods at random in one of the toplevel .st's and, not knowing smalltalk, it was not what I would call clean.
That's given you a false impression. The .st file is only an the export/transfer format to be loaded into another Smalltalk system. Smalltalk code is best read using its System Browser. Think of opening a .docx file in Notepad, and how clean you think it looks compared to editing the file from within MS Word.
Yet not being able to use your favourite text editor is one of the common arguments some people use against Smalltalk, but perhaps there's also an element of them trying cast something "new" (Smalltalk) to them, back into what they already know (as Alan Kay touches on after 1:04:00 in the OP video link)