We store it in clear text - but I would argue not a lot of the data that is stored there is useful. Are they really concerned about their DBAs being able to read a table with millions of entries and seeing the name of a BT or the splitting it uses? But theoretically - yes, names and text fields are clear text and not encrypted.
It sounds like a concern where the best way of addressing is to explain what is stored in the session storage vs. the performance warehouse.
I did exactly that: explain that there should rarely be any sensitive data inside the PWH (unless they start splitting by Credit card or something like that). But they wanted an answer anyway.