The key (and not saying it's bad, mind you) is that the default install has very few services installed, let alone running or open.

So even if Debian and OpenBSD ship the exact same web server, but Debian has it defaulted installed and on, but OpenBSD does not, then a remote exploit won't count against OpenBSD.

All OpenBSD services, including HTTP (httpd), SMTP (smtpd), and DNS (nsd, unbound, unwind), use privilege separation and sandbox themselves with pledge and either unveil or chroot. There's no extra configuration. And the developers dog-food these services; it's why they're in the base system.

How many Linux services use seccomp? Or chroot, mount namespaces, or landlock? If they do at all, it's usually imposed externally by systemd or docker, in which case they usually run with overly broad permissions because there's no integration with the specific application code, thus the AF_ALG exploits in containers. On OpenBSD services continue to narrow their privileges after starting up so by the time an external request is serviced they have only minimal access to syscalls and the filesystem, often only read/write/send/recv syscalls, and if open is allowed only the specific files and directories needed to service requests. Typically even the network-facing daemon accepting TLS connections doesn't have access to the private key--you simply can't do that by running a vanilla service application in docker.

Does OpenBSD have bugs? Of course. The question is, which environment has more trustworthy backstops? The Linux kernel provides all the facilities, but they're not used effectively, for many reasons.

Isn't that a good thing for certain use cases ? If you are building an appliance type thing (say a storage or networking device) then you would want something minimalist you can add only the necessary services on. And arent those the types of devices the BSD (in general) are used for ?

Less attack surface always equals less potential for bugs/flaws/exploits regardless of how good red teaming tools and workflows get.

Now obviously for other use cases Linux could be a much better option.

There was a time when Linux distributions shipped lots of things on by default; OpenBSD bucked the trend and did not. This is less of an issue nowadays.