It quite literally is not an encoder - title should be something like "how to use libav to convert H264 to H265" - libav is the only thing doing encoding here.

I thought it would be about finding a way to losslessly transform a H264 bitstream to a H265 one, and writing a simple application to do it. That would be a very interesting article if it was possible.

Even the title of the page itself is (currently) "FFmpeg libav tutorial - learn how media works from basic to transmuxing, transcoding and more" which is far more accurate.

That's exactly what I expected. Now I'm really curious whether this is possible.

Monsieur Bellard to the rescue please!

ignorant question: are h264 and h265 related in a way that would make this remotely possible?

Basically, no. Basic features like the transforms and loop filters are different. It would cause the result to rapidly differ after a few frames.