This is true. On the other hand, for all we know, Dickerson may have badly wanted to get the heck out town, and was prepared to go play for the Cebu Lions if necessary to avoid South Philadelphia.
Yeah, the specifics are not likely, but the point is that we cannot assume that the Phillies had the option of retaining Dickerson's services for 2020. Many fans tend to assume that the Phillies could sign whatever free agents they want - and that if they don't sign a particular player (especially a player that the fan wanted them to sign), that's because the Phillies "failed" at what the fan assumes would be easy, if only the Phillies "really tried."
This is magical thinking - and it's not restricted to Phillies fans, or to sports fans in general, for that matter.