In his recent biography of Porter, Radical Sacrifice, William Marvel reached a virtually identical interpretation - and, as his source note shows, it is based solely on his reading of the message and not on Sears.
That's interesting, because checking Marvel, he agrees with me.
"At 9:30 that night Porter went back to the Malvern house to dash off a report for McClellan, informing him that the enemy had been driven "beyond the battlefield." With some food, ammunition, and reinforcements, reinforcements, he thought he would have no difficulty maintaining his position, and that he could even advance. "Without these," he added, "we must retire." Commending all who had helped win the day, he hinted at his preference for an advance by adding, "If they cannot reap the full fruits of their labors, they can only regret the necessity which will compell us to with draw." Colonel Hunt and a staff officer took that message to McClellan, whose headquarters at that moment lay near or aboard the gunboat Galena at Haxall's Landing, [
sic] two miles away.* McClellan might have been sufficiently discouraged by Porter's stated wants to opt for retreat had he not already determined on it anyway, once he heard the firing subside. As Porter's note left the Malvern house, orders for the withdrawal were already on their way there, with instructions to demolish the carriages and spike the tubes of the heavy guns that had aided the victory, if they could not be moved. Instead of sending those desperate-sounding orders to Sumner [
sic]**, his most senior subordinate, McClellan addressed them to Porter, his de facto chief deputy.65"
Marvel, William. Radical Sacrifice (Civil War America) (p. 176). The University of North Carolina Press. Kindle Edition.
It is Porter's meaning that is the issue. Marvel notes that Porter's message was discouraging, which I also found when I found what Porter actually wrote. Sears edited the message to make it appear Porter was saying the opposite of what was actually said.
* This is incorrect. McClellan didn't board the Galena until 2300, and is another Sears-ism that has contaminated the literature. McClellan was at Malvern House during this period, about 800 m from Porter.
** Sumner definitely received orders to withdraw in sequence at the same time, but of course he had no siege artillery in his lines. See Meagher's AAR, in which he notes finding a Colonel of McClellan's staff whilst looking for Sumner who led him to Sumner. Sumner then, just after 2100, gave Meagher the withdrawal order. The colonel was probably the order's courier.