The input capacitance is 306 pF at 400 V, so the controller's drive current budget for the target frequency is straightforward to verify.
The through-hole PG-TO251-3 package (IPak, three short leads) is a common footprint for low-power offline converters where a surface-mount part would complicate thermal management on a single-sided board. The 34.7 W maximum power dissipation at the case gives a realistic derating ceiling for a 25 °C ambient in still air.
The 150 °C ceiling is the absolute limit for the silicon — the designer should keep TJ below 125 °C for long-term reliability in continuous operation. The gate threshold voltage maximum is 3.5 V at 70 µA drain current, so a 10 V gate drive provides a healthy overdrive margin that minimises Rds(on) variation across temperature and unit-to-unit spread.