It's just a pun. They're both units of mass, hence there would be mass confusion.
It doesn't work with the pun, but the more confusing part for people would probably be the fact that pounds are used for both mass and force, but in SI, the units are different (kilograms for mass vs newtons for force), though that doesn't really matter for most people.
That's incorrect. They are, in engineering contexts, referred to as pounds mass (lbm) or pounds force (lbf) respectively. The US Customary Unit for mass is the pound (lb) (aka pound mass, lbm)