I didn't say this and there is a 3rd party driver to wire Tableau into BI4 http://toreodata.com/
Tableau folks believe that you can get rid of the BI4 semantic layer if you are using Tableau. This is certainly true in some cases and in other cases, the complexity of the BI4 semantic layer means it may not be practical.
I wouldn't say that Tableau is so much popular with HANA users (though it runs great on HANA, my current project is using it) but rather that because it puts a lot of semantic control in the hands of the user, it is very popular with business analysts and operations people who want that control and flexibility.
There are pros and cons to this, for example that it provides maybe too much flexibility, which means that everyone builds different reports with different numbers.