No, it's not inherently liberal. It's just that I hear it most often from liberals when discussing how Democrats and Republicans both stand for capitalist imperialism. To be fair, that's probably primarily due to the types of discussions that anti-imperialists find themselves in.

I wrote a tongue-in-cheek characterization of the propaganda of the nations we're discussing here, but I'll elaborate further.

There are qualitative and quantitative reasons the equivalence is false.

Qualitative: As the undisputed global hegemon, the modus operandi of the US empire is to maintain full-spectrum dominance over the globe. It does this by exploiting weaker countries and looting their resources for use by American corporations. When a country refuses to submit, US propaganda has to invert reality and depict its victim country as the bad actor in order to justify its actions. Therefore, it inherently relies on distortions. On the other hand, the propaganda of the oppressed nation simply needs to reveal the true intentions of the US empire in order to make its case.

The best example of this is the DPRK. US propaganda frequently invents, out of whole cloth, Looney Tunes-style stories, such as Kim Jong-un executing a government official with an anti-aircraft cannon—an official who later turned up alive, by the way. In contrast, DPRK propaganda is usually centered around telling stories from the brutal war of extermination that the US carried out against it in the 1950s, like its popular museum memorializing the Sinchon Massacre.

Quantitative: US propaganda is voluminous and far reaching due to the empire's cultural hegemony that's concurrent with its military hegemony. Just as the US has military bases around the world that number more than those of any other nation by orders of magnitude, so is its propaganda more numerous than that of its enemies. In fact, US propaganda is even more powerful and effective than its military is. Hollywood movies are popular worldwide. In contrast, the propaganda of its enemies is weak and typically limited in reach to within its own borders, and the language barriers limit the scope of the little bit that manages to extend further than that.

Americans eat, breathe, and bathe in US propaganda. They rarely, if ever, are allowed to see propaganda from enemy nations. YouTube regularly bans channels that speak for China, Russia, or the DPRK. One of the greatest accomplishments of US propaganda is the creation of the perception that this dominance is inverted. They've convinced Americans that they do not consume any propaganda at all, and that the enemy's propaganda is all around them. They've trained them to dutifully ignore or attack any narrative which portrays their enemies in a positive light because it must be originating from its conniving enemies.