In my limited experience, I've found that to be belivable. I've loaded hundreds of 69g Sierra matchkings with all kinds of loads, lengths etc., and consistently got better groups at 100 yards with flat based varmint bullets that I didn't pay special attention to when I loaded them.Flat base bullets seem to be the go to for accuracy at a couple hundred yards or less. Any further than that an boat tail definitely.
Just what I have read anyways. Not experienced it myself though.