Tableau connects to the default View for a SharePoint List - is the default View for the first List filtered to be less than 5000 items? I'd expect to see us hit the List View Threshold error in both cases.
I have no filters on the default view. I created the list in Excel and imported the Excel document to SharePoint. Its a really simple three column list. One column is a counter the other two contain random text.
I just realized in your original post you mention connection to a SharePoint Document Library - as far as I know, the Lists connector was only built/tested for connecting to Lists, so that may be the issue.
As to how the List Connector is getting around the list view threshold, that surprised me. I'd guess the CDATA driver has been written in such a way to pull back the data in batches, or in a way that doesn't trigger the list view threshold error.
And maybe connecting to a Document Library instead of List is what triggered it - potentially as a red herring - in the second instance?