I would say it’s a different category than custom loops - actually building your own loop is a different ball game of complexity, and there is no safety of a warranty.
slapping on extra fans or different fans is a simple change that (hopefully) improves performance for a couple extra minutes of work during AIO install for the end user.
For example, while I generally go custom loop for my main rig, I picked up a Corsair H150i substantially on sale at microCenter when I bought my 2700X. Since I wasn’t intending on building a full custom loop for that rig right that second, I installed the H150i and tossed on 3 gentle typhoon AP 15s that i had laying around in addition to the stock out of the box fans. Total additional work was a couple minutes and additional cost was 0. I assume I got a performance benefit from the extra fans, but I didn’t test without them so I can’t say for certain.