Alonso made his tyres last 46 laps on a street circuit. Stroll made his last 41 laps and still couldn't keep up. That's not a strategy problem.
Lance Stroll started this race on mediums. Fernando Alonso started on softs. One of those tyres degrades faster, loses grip sooner, and forces you to pit earlier.
Alonso finished 15 seconds ahead. Four positions clear. On the worse tyre.