There isn't one BSD kernel like there is one Linux kernel, there are several different ones which might borrow from each other, but are developed independently. A comparison can be found on Wikipedia: https://en.wikipedia.org/wiki/Comparison_of_operating_system...
Dragonfly descends from FreeBSD so it makes sense that it's affected.
The other BSD's have different kernels and vastly different development histories as well.
Some BSDs never allowed debug register writes in the first place, so they were immune.