FBA is scalable.
These people are doing the unscalable part.
"But I was nitpicking whether the word nitpicking was accurate"
I'm not choosing a new word, get out your thesaurus. Then read this: https://en.wikipedia.org/wiki/Principle_of_charity
If you want to trot out the principle of charity, it goes both ways. You assumed people were nitpicking. It was wrong. Every way you want to spin it by changing words, wrong.
And the remainder is not scalable. All of that done by their sellers. As evidenced by the fact that they are not doing it themselves.
I'm beginning to think this thread is being astroturphed. All of you are employing an identical strategy of latching onto minor issues with how things are phrased. A standard PR tactic. And Amazon has a long documented history of astroturphing.
Building all the infrastructure required for FBA is much more difficult than choosing products. The proof is in how many people have done one v the other. Amazon does a decent job on both sides - building the infrastructure, and figuring which products to sell, at scale (they do 100's of billions of non 3rd party sales every year).