I think there is a strong chance they jobbed out the production of that ebook to some kind of agency, likely one specializing in long document production. Adobe InDesign is the most popular application for that now. FrameMaker used to be the best long document publication tool but it looks like Adobe has discontinued development of it.
This PDF file can be done very easily in CorelDraw.
Corel has been doing this stuff through vendors for about 15 years.
In the past they had this capability in house and those employees fed information back to developers, it's gone now so this stifles the development.
I confess that it does not surprise me at all, since since the 2017 version PDF Creation in CorelDRAW has stagnated, the links, for example, have never worked since this version mentioned above! what a pity,because all my shortcut guides and manuals were made and added links in CorelDRAW without error! Now I have to finish the links in Adobe as this one still works fine! And it's not for lack of warnings!