The only problem with that statement is that the US would likely not launch a retaliatory strike and Russia knows it.
The current CIC would not counter launch due to his ideology , a hint of which surfaced in his saying during the 2008 campaign that he felt the US could withstand a nuclear hit. We have become the cheese eating french surrender monkeys.
You have no idea what the CiC would do, nor do any of us, but I might even tend to agree with you on this point. None of us know what pressure would be brought to bear by the military. It would not surprise me to learn that there might be some special circumstances whereby our SSBN commanders have standing orders to launch a retaliatory strike without National Command Authority, such as an instance where the entire Nuclear Command Authority and/or the government is totally decapitated, et al. I would also suspect that the military would rise up and depose him, quickly, if he did not order a retaliation to a nuke strike and they would certainly enjoy the support of the American people in such circumstances. If a nuke strike was launched on the US there would be a retaliation, without a doubt. To believe that it would not is bordering on fantasy.