Yeah, I think the key there is that they're talking about being playable on both kinds of equipment, not the sound itself.
that was really the "end" of mono, really, when you didn't have to buy a mono record just because you didn't have a stereo record player (or from another angle, you coul now buy a stereo record even if you didn't yet have a stereo record player). People were, like, hey, I'll be getting a stereo record player soon enough, might as well buy a stereo record now.
And you know, geez, I had forgotten ALL about that DON'T PLAY A STEREO RECORD WITH A MONO CARTRIDGE - DON'T DO IT!!!!!!!! business, that's like ancient record history now. But yeah, that was a thing. All the record club ads, you had to select stereo or mono, it was a very real thing.