We know that ADHD is overwhelmingly genetic.
Thus it makes sense for a disproportionate amount of ADHDers to be raised by single parents, as sadly but not surprisingly, women with ADHD (and yes, they very much exist, and are indeed underdiagnosed) are at a higher risk of unwanted pregnancy (due to impulsiveness, a vulnerability to addiction and escapism, and problems with routines - like contraceptives) and ADHDers in general more likely to get divorced (because, well, we are not always the easiest people to deal with, especially if our condition is not known). But here, the child’s hyperactivity is not caused by the mother’s singleness. The correlation is because her singleness and her child’s ADHD have a common course: her own ADHD.
Anyways, my brother and I am very much ADHDers. And you know what? My parents were married until my father’s death when I was 19. Also, we definitively have it from my mother.