Although biscuits are designed to expand in the slot, the slots also have a bit of play in them. So it's a s much the glue causing the swelling as any other factor, which means that a spline can also cause swelling. Experts advise to wait a few days after gluing to allow the water to finish evaporating out of the wood before sanding and finishing. This lets the swelling go down so that it isn't a problem. I've read of people who sanded right away to level and then wound up with a dished surface days later.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.