It's a highly specific HT4 agonist, I'm not sure why you'd be more likely serotonin syndrome by combining it with MDMA vs MDMA alone - it doesn't affect release or transport, nor does it have have significant activity at other binding sites.

Am I missing something?

No, I'm just parroting the common wisdom of the drug crowd. But looks like there might not be much to worry about indeed: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5931730/