It's using Katex, which is smaller and faster: https://katex.org/

Yes, I'm using katex because I found the API easier to work with.

Are you rendering the LaTeX on server-side or on client-side using JavaScript? I am asking these questions because I find MathJax code in your website:

  <script type="text/plain" cookie-consent="strictly-necessary">
    window.MathJax = {
      tex: {
        inlineMath: [['$', '$'], ['\\(', '\\)']]
      },
      svg: {
        fontCache: 'global'
      }
    };
  </script>
  <script type="text/javascript" id="MathJax-script" async
    src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-svg.js">
  </script>

Actually, that code should be deleted. At one time, I was experimenting with MathJax but don't use it anymore.