A lot of that decline is from the price of crude oil.
Which actually strengthens his point. Russia does not have a diverse economy. The vast majority of its income is derived from oil which is a remarkably volatile commodity. During the same period the US also had some issues as a result of the growth of the energy sector but our GDP did not shrink at all because our economy is so varied.
Russia has been a major oil and gas producer for well over 100 years. But it's thanks to Putin that it produces almost nothing else. It doesn't take a rocket scientist to realize they desperately need to diversify the economy. And in Russia, doing so would be pretty easy. Putin and his thugs are too busy looting and strongarming control of private business to tolerate a normal economy.