This does not mean using you smartphone on the toilet causes hemorrhoids at all. It is more likely that people with constipation know they will be in the bathroom longer so they bring their smartphones, and constipation IS the leading cause of hemorrhoids. Correlation is not causation.
Fwiw, the two feed into each other. I read this same thing somewhere else before and have since tried to not take my phone with me into the bathroom. I started doing that because I would take a lot of time in there. But once I have my phone, if I start reading something a bit too long, I could easily spend 20 mins in there before realising it.
The position of sitting in the toilet without support for the pelvic floor adds extra pressure to the related vein and vascularity, creating hemorrhoids.
Another trash conclusion from a trash study.