Calico is installing routes in the Linux kernel. Those routes are pulled out and distributed using BIRD. BIRD can do OSPF instead if you'd like.

All Calico cares about is that routes are distributed across various systems, they don't necessarily care how you do it (configure BIRD however you'd like).

BGP is surprisingly simple and easy to set up with BIRD. Setting up a route reflector with local hosts on the same L2 all peering with each other and suddenly you can route whatever IP's you want by announcing them to your peers.

Why do people think BGP is complicated?