Oh, I could see how this could be misleading. So the open-source tech you linked to is outdated -- we moved everything from the client-side to our own infra (because we wanted to do things like open a port). We intend to open source the backend infra too but it takes time for the protocol to stabilize.
The "open-source" section on the homepage is intended to show that some open-source projects (for example Jest https://facebook.github.io/jest/ were repl.it is embedded on the homepage) use us as a playground, for bug reports etc.
As for the logos they are either users or customers of the the Classroom product: https://repl.it/classroom
Google, for example, uses the Classroom product to spread programming education. I think they have a program where they send teachers to schools to teach computer science. And there teachers wanted to use Repl.it etc.