Technically, I see the point about the division between the Sunset and the Parkside as corresponding with the 94122/94116 zip code division, but there are a couple of other considerations worth noting.
Emotionally, I think the area bounded by 37th Ave., Ortega, Quintara, and 41st seems to serve as a transitional "island" between the two districts, at least west of Sunset Blvd. It could be because the sand dunes were there so long serving as a sort of "no-man's land".
Also, the park there is "West Sunset Playground", not "West Parkside".
The Sunset kids attended A.P. Giannini and "Sunset" Elementary, both technically located in "the Parkside", if the Ortega boundary is indeed correct.
Then there's South Sunset Playground, right in the heart of the Parkside!