Hmm. Fifth Corps, is it? Wonder if they were lying about the message being from Barnes, or if it was a division commander passing the rumor? Here's another source dealing with the same brigade which describes a staff officer sitting a horse along the Fifth Corps' line of march, retailing the story to each colonel as they passed. The site author's footnote links are broken, sorry about that.
Here's Henry Hunt who notes that he first heard the rumor on Culp's Hill, and was of the opinion that it had originated with the Fifth Corps the night before. Here's another source from Vincent's Brigade, 16th Michigan.
There had been a previous rumor in Second Corps on or about June 28th, according to William Lochren of the 1st Minnesota. This rumor apparently also swept through Third Corps on the same day, to judge from this account from the 4th Maine of Ward's Brigade. With the limited resources of Google, I'd guess that the post-Hooker-demotion rumors of June 28th had inspired the unnamed staff officers of Fifth Corps to spread false news during the forced march of July 1st for the purposes of morale. Webb's invention of an independent force under McClellan in the enemy rear on July 3rd would seem to be third iteration of this rumor, designed to steady his brigade when the sounds of fighting in their right rear broke out - from Culp's Hill, not that Webb or anybody else in his brigade would know what it signified.