If you check out the livestream on youtube, you'll see that he does the writing in Bear (Mac/iOS only app). Presumably there's some custom code to export everything and build the HTML with backlinks, but as mentioned in the link posted here, this code is not public anywhere.
He has shared part of his toolkit, namely the exporting and syncing of the Bear notes, and “link-janitor” for the backlinks [1]. Although I wouldn’t recommend it in general - it’s a brittle prototype. And right now there are better tools out there (linked above) many of which appeared in the last few months.
