A bit more info on this after checking with the developers…
KNIME is using a standard XML writer that comes as part of Java. It has the quote character for XML attributes hard-coded, and there apparently isn’t any option to change that. That said, KNIME writes valid XML documents, and it’s not clear to us why Tableau doesn’t accept this format. (Sorry, I know that’s not a very satisfying answer.)
In the future we will be updating the XML Reader node to address some other items, so if there’s a different Java library out there that supports both single and double quotes in export, we can see about integrating it.