My "Mingo" John Thomas had a brother by the name of David, and the David Thomas in your story could well be him, as John was also Welsh.
I find John and David on the 1793 & 1790 Mason County tax rolls, which also tracks right as John did not bring his family to Champaign County, Ohio until about 1806.
Do you have any knowledge of this Thomas family when they were in Washington County, PA.
I believe John's parents were Owen Thomas & Mary(Polly) Hardin, but have been unable to totally document this to date.
Too bad these early settlers didn't leave a few more records of their lives.