Yea, well that was my first though, but then I though - why would chrome even allow any website to just arbitrary check which extensions you have installed.

So I checked the scripts and at this line the script is showing

async function fetchExtensionInfo(extensionId) {
  return new Promise((resolve) => {
    const url = `https://chromewebstore.google.com/detail/$%7BextensionId%7D`;

So I thought maybe they were calling the chromewebstore foreach plugin, and if you have an extension already installed, you get a different response than when you don't - or something.

But I suppose I'm wrong and for some reason a site can just ask the browser internally which plugins are installed