You can find papers that say whatever you want if you look hard enough, all written by people who study fish parasites. I prefer the ones that say Ich cysts are all the same size and shape, because in aquaria that’s nearly always the case.
Epistylis and a few other skin infections are very often misdiagnosed as Ich, and papers like the one you linked only exacerbate these misdiagnoses, imo.
It is true that Ich meds will also kill epistylis, since they’re both protozoans, and it is a valid treatment if in doubt, but with epistylis you also have a bacterial skin infection to treat. Treating that means you don’t need to kill the epistylis, which normally lives harmlessly in tanks and only attacks bacteria.
A fish arriving with bacteria on the skin will suddenly get epistylis if it was present in the tank.
(If there’s any white ‘fluff’ or little white tufts on the glass in any low flow area, that’s epistylis).
I'll concede that it's possible for an epistylis infection to occur in a new fish if it already had a bacterial infection upon introduction.
However, picking and choosing which narratives you like best is not appropriate if your aim is to seek the truth of a subject. Many of these papers discuss fish which are kept in laboratories. Those are definitely aquaria too, but I assume you mean home aquaria. Do you have sources that state ich is nearly always uniform in size in
home aquaria as opposed to laboratory aquaria? I'm genuinely curious. What distinction is there to be made between home and laboratory aquaria that would meaningfully affect the growth of a well-known parasite? I'd genuinely be interested to read the papers you've found which say ich cysts are all the same size, because if there is a distinction between ich found in home aquaria vs laboratory aquaria, that's quite notable.
One thing I can provide are images. I'll include one from Buchmann (2020) of an
Ancistrus with ich.
I'll also include a couple from a non-literature source. These are definitely ornamental fish kept in home aquaria.
The second two cases are very likely to be ich. In the case of the
Ancistrus, it is known to be ich because it was in a laboratory setup where it could be tested. In all of the photos, the cysts present with a variety of sizes, and are not perfectly circular. At the end of the day, epistylis is something that can really only be diagnosed with certainty by using a microscope. The same goes for ich, to be honest. Microscopes are expensive, and so most hobbyists don't have them. But ultimately, you can't make any firm diagnosis from a photo. Without microscope images, it's a guessing game. Sometimes it's educated guessing, but it's guessing nonetheless.
Buchmann paper:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7507210/
Source for the second two photos:
https://fishlab.com/freshwater-ich/
(I'm not vouching for the fishlab source, just using it for a couple of its photos. It's imperfect, as most online articles are)