It's true. This Liverpool team (and City team) would beat any Man United team.
The levels of fitness, nutrition, tactical knowledge, training methods (like spatial awareness and making perfect decisions in split seconds for Liverpool especially), is vastly superior now. The psychological aspect also has moved leaps and bounds ahead.
The amount of running Mane Robertson TAA Henderson Wijnaldum etc do now is ridiculous compared to back then. They would open up so much space and run rings around United alone, ignoring technical skill, or tactics etc.
It's really unfair to compare the two because football has progressed so much since then. It's like comparing a 90s United team to a 70s Liverpool team.