[bbertelsen]

What did they use to build this site? I could have sworn I saw what looked like LaTex when it was loading?

[Cieplak]

Looks like quarto. The LaTex you saw may have been MathJax.

    $ curl -s https://manifestai.com/blogposts/faster-after-all/ | grep generator
    <meta name="generator" content="quarto-1.3.450">

[JoelEinbinder]

Developer tools point to MathJax https://www.mathjax.org/. If you disable javascript you can see some LaTex.