Frustratingly (to me, anyway), the article fails to define what 'UPA' stands for. I looked it up - United Productions of America, apparently. (I guess it was assumed the intended audience knew that.)
Think about the differences between an early Disney feature and something like Hanna Barbera or The Simpsons. The latter two are examples of "limited animation," which is usually a cost saving measure: draw fewer things, take less time, spend less money.
UPA was a pioneer in limited animation, but as an artform. They leaned into their constraints and created something beautiful from them.
There's way more here than I can write in a comment, but if you have an interest in animation, art, business, or working within constraints, you may find it interesting to spend an hour or two learning about UPA. (I haven't read it, but the linked book sounds like a good source.)