The article is somewhat unclear about the rationale behind the dosages, whereas the paper itself provides a better explanation:

"The rationale for dosing regimen utilized was based on a number of factors. First, we sought to model high-dose used in a clinical study for chronic pain, where patients were administered a psychedelic dose (25mg) of psilocybin. Using the standard allometric scaling method, a human dose of 25 mg of psilocybin translates to a mouse dose of 5.14 mg/kg; this informed our starting point for dosing. However, mice exhibit a significantly faster metabolic profile for psilocybin compared to humans, leading to a shorter half-life and more rapid systemic clearance of psilocin; the elimination half-life of psilocin is ~0.9 h in mice vs 1.8–3 h in humans. Due to this rapid clearance in mice, a higher dose of 15 mg/kg was selected to ensure sufficient systemic exposure comparable to those observed in human clinical trials."