Oh yeah, your second interpretation is spot on. Figures 1 and 2 kind of hint at this. For CREB327-WT, the GSK lane alone is blank, but the PKA + GSK-3 lane shows more phosphorylation than just the PKA lane. This is implying that CREB327-WT phosphorylation seems to happen in sequence with GSK-3 only being able to phosphorylate CREB after PKA.I think I am reading too much into the wording. When I read, PKA can only partially activate CREB327, I thought it meant that somehow PKA doesn't complete it's phosphorylation on the CREB327, like maybe one of the phosphate groups became nonfunctional somehow.
I didn't read it like, PKA phosphorylates CREB327 to activate it, but not to the extent that a normal activation would be by both PKA and GSK-3.