I don't know where truth actually lies, but Howard did do amazing things. I've always thought there are three things that could make you a famous for your skills with a bow: 1)hunting success, 2)tournament success, and 3)trick shooting. Quite a few archers are famous for one or maybe two of those, but Howard was famous for all three.
Now I do think there probably has been some embellishment of his feats just because of the time period when he did his thing. Sensationalism during that era of Hollywood drew folks in. And there were probably some staged shots on animals because the camera equipment sure wasn't what it is today.
Nevertheless, even if you took away half the animals he supposedly killed with a bow and half the tournaments he won, he would still have more accomplishements than 99.9999% of archers out there.