I don't know why you think this particular Linux-libre bug prevents execution
of proprietary code: replace existing libre firmware. Heck, I'll give you a
real-world example: there's an old, proprietary firmware blob that can be
loaded onto the Atheros devices Think Penguin sells, and Linux-libre will
happily load that if it's available.
On the other hand, if you develop a libre replacement for a proprietary
firmware blob, Linux-libre will not use it, because it isn't aware that it
can. Do you remember back when Think Penguin's wifi adapters first got
certified by the FSF? There was a period of I think a month or something
where they were completely useless in Trisquel without completely replacing
the kernel because of this bug. This isn't a critical problem, but it's still
a problem.
So no, this bug in Linux-libre is not a security feature.
