A note and reminder about Relays in general are that they are more of a convenience in the protocol than a hard requirement. The “firehose” API is the exact same on the PDS and on a Relay. Any service which subscribes to the Relay could instead connect to one or more PDS instances directly.
The code for their instance isn’t open source, so it’s legitimately impossible to host another relay. So until that changes, yes, it’s a lie
Isn’t this the source for the relay? https://github.com/bluesky-social/indigo/tree/main/cmd/bigsky
And even then the readme says:
And the PDS source code is here: https://github.com/bluesky-social/pds
EDIT: The PDS source is actually here: https://github.com/bluesky-social/atproto/tree/main/packages/pds the other link is for self hosting.