Here is a more definitive source, from olaf's saga. Around page 195 they make a direct reference to spjall being known for axe-throwing.Quick google search of the saga, Harðar saga og Hólmverja - Icelandic Saga Database
It's chapter 33, and I still don't really get it. I get the feeling part of it got lost somewhere, or there's a translation issue, but I'd have to research it more to know.
Olaf's Saga