Even a fully loaded kernel with loads of drivers isn't that big. And not all of it has to be resident in memory at all times. Code in general is miniscule compared to data. And most of a kernel's data isn't baked into the executable. And this kernel in particular has very thin drivers that only abstract real devices to generic device class interfaces that userspace has to deal with directly. That's the part that's inspired by exokernels and hypervisor paravirtualization. That means that drivers for this kernel will be even smaller than those for other ones like Linux.

OpenBSD removed support for loadable modules. Hardware today is big enough that compiling everything in is fine, and we don't need a ton of fiddly code to put a special-purpose linker into the kernel. Saving a bit of memory isn't worth the risk.