Dana,
I think that you have read the PMBOB v5 DFD figure 5-10 incorrectly. The Validate Scope is the output of the WBS process along with Project document updates. The Scope Baseline shown along side the dotted line is to inform you that each of the processes outside of the grey area of DFD figure 5-10 references the Scope baseline and that includes the Validate scope process as well. In other words, the Validate scope process measures performance against the Scope Baseline.