There are many network cards that don't require blobs, for instance. In fact, most don't need need firmware uploaded to them. The list of kernel drivers far exceeds the list of firmware blobs in linux-firmware.
You don't need coreboot for them to distribute a free kernel. You don't need coreboot for them to distribute a free OS. You would need coreboot if you wanted your system to be 100% free, but that is separate from what they *distribute*.
In fact, if what you were saying is true, basically no one would be able to run the linux-libre kernel, yet people do. How do you reconcile that?