If you're on Android, there's an accessibility option in Firefox and Chrome that allows you to always zoom on any page, even if the website normally prevents it. On Firefox it's under Settings -> Accessibility -> Zoom on all websites.
Hey, I'm the author of this site!
You're on Android I think, based on the other comments here? I only have an iPhone to test, but I can pinch+expand the site on my phone no problem. Was not intentional to have blocked it! Sorry, I'll see if I can work out why.
That makes some sense actually if iOS ignores it. Whenever I come across the behaviour I wonder why, but if it's just a copy/paste magic meta tag which goes unnoticed by most browsers then I can see how that could proliferate.