I was not aware of this but logged in to my Upromise account and it does seem to be true. Link takes you right to the Marriott website and you can enter corporate codes into the search box.

However if it is not working for you what is the point?

Let us know if you work it out. I may just try a booking for the fun of it, nothing to lose I guess as long as I still get stay credit and Marriott points.