No, it is clearly not as the pigeon flies. That is incorrect.
When I click on my local subway stop it seems entirely accurate, and the stations don't form any kind of circle at all. In fact I can't find any subway stop origin that seems to form anything even close to a circle.
Also, according to the "about" it's calculating from actual subway schedules.
And it has nothing to do with street grids. It's subway trips, not walking.
The station-to-station part does indeed use subway travel times, but the website says "Hover over a station to see how much of the city is accessible within 40 minutes by subway and walking."
And the "walking" part clearly shows a perfect circle around each destination station, so it is 100% doing an as-the-pigeon-flies estimation for the walking.
"Isochrones are manually calculated using turf.js assuming 1.2m/s walking speed after the subway trip. These are simple buffers around each station/prior isochrone and do not take the street network into account."
Other people already corrected you, but I feel like being obtuse so here it goes. The circle goes from Jamaica, along Queens Blvd, down 8th Ave to W 4th St, along the F line to Jay St, down Fulton St to Broadway Junction, past Cypress hills on the Nassau line (J/Z), and finally returns to Jamaica. This loop is possible without adding any additional tracks or switches to the system. Thats how the pigeon flies. Checkmate.
When I click on my local subway stop it seems entirely accurate, and the stations don't form any kind of circle at all. In fact I can't find any subway stop origin that seems to form anything even close to a circle.
Also, according to the "about" it's calculating from actual subway schedules.
And it has nothing to do with street grids. It's subway trips, not walking.