Bat may not have been in the vicinity at the time of Ringo's death, but he was certainly with Wyatt in Colorado and/or in communication with him during that summer and many times thereafter until his permanent move to New York in 1900.

Until around 1920, when Wyatt began spreading the story that he killed Ringo, Bat is the only source I am aware of that even hinted at that possibility (other than John Yoast's exclamation when he found the body). The fact that it was Wyatt's best friend writing in 1907 and 1910 who suggests a return trip to continue the vendetta is, at the very least, curious. Perhaps it was just a throwaway line, on both occasions, maybe even added by Alfred Henry Lewis for dramatic effect. Maybe Wyatt was lying about it (if he was lying) to his closest friend much earlier than he told anybody else. Maybe it was the inspiration for Wyatt's own version he began to share ten plus years later.

Still, in any cold case investigation by modern law enforcement of an open death (the coroner's jury didn't rule on the cause), these public statements by a close friend of one who at least had a motive, however tenuous, would be looked at seriously rather than cavalierly disregarding them.

In terms of my "pro-Earp bias" I leave you with this:

https://nediscapp.herokuapp.com/discussion.cgi?disc=39627&article=18473&page=0