Sounds like everyone had a good day. I caught it at Victoria at 1524 and rode in the leading car to Monument. The train op was obviously in the spirit of things as he blew the whistle often and made announcements about it being the last D stock train etc. A bit of history.
The website also says that you can buy longer period travelcards without registering as long as you have a contactless and Oyster account. So it seems that registration is really only for those who don't have access to the internet.