It is not a true Starbucks...at least it wasn't when we were there last. It was a coffee cart that features Starbucks coffee. My DH, who is a coffee nut, prefers the Kona blend over at Poly. We'd stop in the mornings on our walk from SoG to the Poly to use the monorail.
ETA: You can walk onto SoG property at any time. Driving through the gate requires a reservation or tee time.
The cart does make lattes, cappucinos, etc., so that was enough Starbucks for me. It was still leaps and bounds better than anything Disney served, except for the Poly.