No. The bathroom above is not a vent for the downstairs.I was told that the stack from the upstairs bathroom, which is about 8 feet away from the floor clean-out, would serve to vent the new bathroom.
You will need to provide proper venting for the bathroom in the basement, apart from the upstairs plumbing.
You may be able to do this with an AAV that is accessible for replacement and is open to air. Either exposed or with a grill that allows air access.
If you open up the floor, you may be able to wye off the horizontal there.