And the author is not documenting them either, just announcing his new niche library. It is not like disassembling a few functions to prove that they exist is dark magic. I just don't see any value in the article.
There are some numbers here on the performance improvements he’s managed to make.
It's explained in a bit more details in this original blog post written before the library was optimised: https://atomic14.substack.com/p/the-fastest-esp32-jpeg-decod....
It's very easy to forget what a range of MCUs there are, from very puny, to very capable. For example the Espressif range of MCUs - which you'll find in all sorts of consumer products - are very powerful. Couple that with a lot of cheap SPI based display modules and you very quickly start wanting to show images.