The abstract you linked, pasted below, seems to say otherwise. Placebo groups report same SE frequency.
“Across 684 randomized controlled trials, reported SEs were infrequent. Although dose and duration tertiles were statistically associated with study-level side effect reporting, the effect sizes were uniformly small, events were infrequent, and the reported symptoms were primarily mild and nonspecific. No consistent exposure–response pattern indicative of clinically meaningful risk was observed. Adjusted logistic regression and frequency-based analyses showed no consistent dose- or duration-dependent increase in SE risk, with placebo groups often reporting similar or greater SE frequencies at the study-reporting level. CrM appears to be well-tolerated and, at the study-level, does not increase the risk of gastrointestinal, renal, liver, musculoskeletal, or other SEs compared to placebo, even at high doses or longer durations.”
1) Creatine is safe, "creatine supplementation is safe across a range of doses, durations, and populations according to human trials".
2) Stomach issues are a commonly reported side effect, "Gastrointestinal issues were the most frequently reported side effect... Most notably, gastrointestinal distress is a commonly reported side effect, and those supplementing with creatine may need to divide the dose into smaller boluses to alleviate it; however, it is worth noting that this side effect was not persistent"
3) It is not clear if the side effects are caused by creatine, many of the issues can be explained by baseline issues rather than caused by creatine, "No consistent or clinically meaningful dose-dependent increases in side-effect reporting were observed across models; even at higher doses and prolonged durations, reporting remained low and largely comparable to placebo at the study level"
The general school of thought behind the belief 'Creatine HCL is easier on the stomach" is:
1) HCL is much more water soluble and (likely) is more easily digested.
2) less creatine HCL stays undigested.
3) the dose needed is smaller.
4) anecdotes seem to support HCL being easier to digest.
To the best of my knowledge, there are no peer reviewed studies of monohydrate vs HCL that look to establish which is easier on the stomach.
[1] https://www.mdpi.com/2075-4663/14/4/137