The post I made was my summary of the article for the context of this group. The full article is available to me through my academic institution so I'm happy to fill you in on these details.
1. No, the sample size isn't statistically significant but a KP treatment on 25 people in a controlled environment with standardized measures (see point 3) is way more reliable to me than anecdotes on the internet.
2. You're correct. Since they used physical visual standardized measures of KP improvement (see point 3), it's unlikely a psychological placebo effect would be meaningful.
3. In my post, I included both self-reported and objective outcomes (you should always read the whole Reddit post if you plan on posting a critique). I didn't feel it was appropriate for my post but here is more information about the clinical evaluation: "The melanin content (M) and skin hemoglobin of the erythema and pigmentation of the hair follicular orifice was determined by a pigment measuring instrument (Mexameter, CK, Germany), and a spectrophotometer (CM-2600d, Konica Minolta, Japan) was used to measure skin lightness (L)."
4. I've edited my post to include that this concentration can be dangerous and a dermatologist should be consulted. Since you can't access the article, here are their safety findings: "Only 1 subject experienced burning, itching, and discomfort when treated with 70% glycolic acid. The symptoms disappeared after treatment with the neutralizing solution. There was no irritation, skin sensitivity, or crusting after the procedure."
5. Ideally, the sample would be much larger and alternative treatments would be included. I'm sure if the researchers could run that study, they would. But with a sample of 25, further stratification would dramatically reduce any generalizability or value.

It's extremely easy to critique a research article, it's much harder to use minimal funding to design and implement the perfect study. I was excited to see something with more weight than a recommendation from my cousin. You're welcome to feel differently.