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 site says "accessible within 40 minutes by subway and 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.
Click on Rockaway park. A pigeon could reach those islands, but a walking person could not.
This is still a great visualization.
"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."
It's all bravado commenting on something you're working on in the comments of someone else's actual working site. Let's see the comments from a link to your site? Thought not