Seems logical… if you ejaculate regularly then any sperm you have are likely to have been produced more recently, and hence are probably less damaged. Not sure what the “shelf life” of a sperm cell is, but if all the sperm you’ve got are from the previous 24 hours then they’re going be fresher than if you’ve got some week old cells mixed in there.

Possibly also a hormonal connection – frequent ejaculation leading to ramped up sperm production because as far as your body’s concerned frequent ejaculation means you’re having a lot of sex, and to maximise your genes’ propagation into the next generation you need to have a ready supply of plenty of sperm.