In a strictly literal sense, this is true -- it's not within his power to determine what Moody's or S&P does whenever this standoff ends. It's also worth noting that what the ratings agencies do is often inscrutable, can be arbitrary, and is not immune to outside pressures, as we saw in the wake of the 2008 financial crisis. But it's a stretch for him to suggest his actions have no influence over what the agencies do: if he'd agreed to a "grand bargain" with President Obama, he'd confidently be able to predict the country's current rating would stand. If his plan didn't portend yet more debt limit brinksmanship in a few months, he'd be on firmer ground than he is now.
Boehner did tacitly acknowledge that his plan might change, though. Pressed whether his debt limit bill is a "take it or leave it" proposition for Democrats, Boehner would only say, "we have a reasonable responsible approach, there is no reason for anyone to object to it."
Asked whether the House would be in session this weekend, after (presumably) passing his plan, he said "sure."
So we'll have another round of this.