As far as I understand, you could redistribute BSD under the SSPL, since the BSD license doesn't prevent relicensing as long as you keep the BSD license as well.
And besides, there's a difference between the practical impossibility of building a kernel and the idea of the actual impossibility of complying with the license terms.
That said, I think the argument that the SSPL makes it difficult enough to use the software (due to the bookkeeping requirements of tracking every piece of software that interacts with the SSPL software) has some merit.
