Not quite the same, but my startup, Formgraph[0], also does public, real-time collaborative drawing. I even did a similar write-up about the stack behind it the other day[1]. Looks like they're relying on Cassandra and Redis. I went with RethinkDB. Should probably do a write-up about the front end in the future.

[0] https://www.formgraph.com

[1] https://medium.com/@JohnWatson/real-time-collaboration-with-...