That's not entirely fair. Adobe has openly released a comprehensive description of the format which is, as far as I know, accurate. The problem is that the format itself is a heap of features piled on year after year with apparently no regard for doing things consistently.
Older illustrator files are in fact, EPS files. This is somewhat tricky since, an EPS is not actually so much a data format as it is a turing complete programming language. Sooo... yeah, who knows what black magic they did to pull off reliably reading and writing it.
You can write an EPS file by emitting your list of shapes without using the turing-complete features of the language.
Not that hard, at least conceptually.
But seriously, this kind of back-compatibility horror is the hallmark of success, where success means enduring.