This is complete nonsense. US has never, ever faced a remotely "modern" adversary on the battlefield, so you cannot judge any potential air superiority potential. Even tiny Serbia with heavily outdated and vastly underpowered army was able to shoot down a state of the art, cutting edge F-117A Nighthawk during the Bill Clinton bombings.
However, you may personally feel on the matter, in case of Russia, any judgements should be reserved, as Russia is not fighting Ukraine in Ukraine. They are fighting the whole world. I doubt ANY country, including US could achieve any air superiority if it was fighting the whole world. All factors considered; Russia is making titanic strides. Only someone completely uneducated, would deny that if not for the immense, incalculable amounts of outside aid, Ukranian regime would fall a long time ago.