From the demo, the payment overlay seems integrated directly into Safari. Hopefully there's an OS level API that Chrome et al. can hook into, otherwise it's going to be fairly limited in adoption.
That said, I would have much preferred that it work on any browser on any OS and only require an iPhone, but I'm interested in Apple Pay online enough to change some habits.
I run an online store, about 45% of transactions happen on a iOS devices. I will absolutely be implementing Apple Pay.