1) Because the published version of Myron Loop's diary references a letter home that corroborates his story.
2) Because the 68th Ohio, a veteran unit, took almost no casualties, which supports the story that they remained unengaged for the duration of the battle.
3) Because there is no primary source that reports seeing them run from the battle.
The origin of the idea that they skedaddled is Ed Bearss' trilogy. This assertion was picked up and repeated in almost every subsequent account. I'm amazed that the lack of casualties in the 68th OH didn't catch anyone's critical eye.
Ed Bearss' only source for the assertion that the 68th OH ran is Oldroyd's account stating that Logan was riding the line screeching like an eagle, turning the unit to Oldroyd's right back to their place in line. However, the unit to the right of the 20th OH was the 20th Illinois, the 68th OH having been ordered to the extreme left by this point.*
The O.R. for this period has huge gaps. Because of the fluid nature of the campaign, many of the reports were never filed and many were written weeks after the fact. I think a lot of the problems in the Bearss account is that he relies largely on the O.R., and the gaps are filled in with quite a bit of supposition.
The assertion that the 68th OH fled at first contact is simply the most easily and definitively refuted of these suppositions.
* Oldroyd as a source for what was going on around him is problematic, because he was face-down in a 10-foot deep creek bed. Members of the 20th Illinois report being ordered back to the fence line for cover ... not fleeing the scene as reported by Oldroyd. Members of the 20th OH report being ordered to charge into the creek bed, which would mean that Manning Force actually overshot the unit to his right, which was still working its way through the woods when the 7th TX charged. Later, the 20th Illinois was ordered to fall back to the fenceline for cover, widening the gap between the 20th OH and the 20th IL.