Do people actually use OpenBSD over FreeBSD for use as a router ?

Yes, lots of large providers really want defense in depth at the router/firewall/important services level.

They want separate OS's in case a giant CVE or bug comes out for platform X but isn't on platform Y. So multi-platform core services is a key component of systems that need to stay up and alive long-term. Things like Routers, Firewalls, DNS, BGP, etc. Use different software and hardware stacks.

We use FreeBSD and Debian @ work, though we are playing around with Nix and Guix now to maybe replace the Debian someday.