It was only ever true by an extremely specific, now outdated, definition of loudness. Basically commercials back then would have the audio extremely compressed, such that they were always at the maximum possible volume allowable by your system at a given volume setting, whereas content mixed to be enjoyed would be mixed such that there are louder and quieter moments, which is both gentler on the ear and more dynamic.

You know how movies are mixed such that they have really quiet dialogue and big explosions are like 4-5x as loud? Commercials are in explosion-mode the whole time.

So yeah, commercials are mixed louder than the rest of the content on purpose, just to try to snag your attention. If they could, advertisers would come into your house and turn the volume on your AV receiver/soundbar/tv/computer/phone way up themselves.