I think that’s because of where the city is defined. SF is just SF- it doesn’t extend that far south and doesn’t even include the east bay. NYC on the other hand includes big areas like east Queens and Staten Island that are effectively just suburbs.
I’ve lived in both and if you’re sort of near the core then NYC is much, much more walkable than SF.
SF is walkable if elevation is not a problem for you. Some slopes are so steep that it's like hiking up a trail.
At least it isn't like San Jose or Newark, where sidewalks just aren't there along some roads, in residential area, or suddenly cut short in the middle of a block where you have to walk back to the previous crossing for an alternative route. And there are _so many_ dead ends without warning. So many.
Depending on the boundary of the city, it might not be an apple-to-apple comparison.
I wonder if NYC will rank higher if Staten Island is excluded. Not that I have anything against it, just that judging from the heat map it is less walkable than the rest.
It's similar in Canada too. Toronto is the megacity (1998 amalgamation) including some suburbs; Vancouver does not include Richmond; Montreal has weird holes like Westmount.
Discussions of urban density often require nuance because density/walkability/access to transit/etc. is very dependent on where political boundaries are drawn--often for fairly arbitrary historical reasons.
ADDED: For that matter, the other Boston neighborhoods out that way are not especially conducive to living without a car either although they're dense enough in spots to walking to some things.
Based solely on the metrics of the site, walkability, bikeability, proximity to transit, based entire on map data… sure! The reality on the ground of course suggests walkability is… not great. Better than having a car there though.
I’ve lived in both and if you’re sort of near the core then NYC is much, much more walkable than SF.