He says this in two different places. First, as long as capital has a return greater than zero, the interest rate will not fall below it. My guess is that he was referring to the "real" rate of interest which takes into account inflation. The only time I can recall negative real interest rates was during the Carter years when the prime rate was 21%. I can't decide if this guy knows anything or is just sloppy in the way he writes it.
I remember the interest rate on the house mom and pop bought in the mid 70s was like 18 percent!!
Seems like a lifetime ago.
God forbid it ever goes that high again, housing prices will plummet.