The link is at the bottom of the site, but it's pretty subtle, and I think these watches have the same mods.
Edit: These have NFC, but no SD card, so maybe they will show up with enhanced storage later, but anyone wanting the NFC features with the watch it looks like they are available.
Edit Edit: The NFC chip in the ones for sale is not tap-to-pay :( still neat though!
However, the relevant authorities might take a dim view of repurposing the transit card in this manner and probably have certain restrictions in place, to deter people from defacing their property. On another note, the author is using a NTAG213 on the watches sold via the shop and Oyster is Mifare, from what I can remember. There might be issues with response times regarding the proximity to the reader, if you wanted to experiment.
I could dig that up if it’s helpful but in general I think it’s pretty straight forward, just some risk of destruction :)