at the time he wasnt but russias intervention in ukraine really escalated the situation. medvedev also had much better relations with obama when he was president than putin did.
The main ways in which Russia has become a major foe of the United States are:
Russia's siding with Assad in the Syrian Civil War (began in 2011)
Russia's invasion of Ukraine (began in 2014)
Russia's interference with American elections (began in 2015)
None of those things were happening in 2008. Russia was doing nothing that would have made them America's "#1 Geopolitical Foe." A five-day conflict with Georgia was pretty much the height of antagonism with the USA, and it wasn't even direct.
In fact, in the early years of the Obama presidency the relationship was mainly characterized by a defusing of the whole Poland missile issue, coming together to sign the START treaty, and coming together to work on the Iran treaty. So Obama was right.
It was really the Ukraine issue that caused everything to blow up.