This may host it live, but all presentations are recorded and will be here soon.
If I sign up for a service using Apple Sign In, is then possible to sign into my account from a Windows etc machine subsequently?
If you have used Apple's service - which is completely opt-in - you could always go into an account and change the email you associated with it if they let you do that. And if you don't, this would be in the ballpark of switching email providers - yup, you'll need to change some accounts, because you've decided to no longer pay for the service from the Apple email forwarding.
The "works in any browser" and "apps running on other platforms" parts seem to suggest it will work fine on Linux or Windows machines or even Android devices.