I guess the distinction would be if someone wanted to upstream a change, not fork it, through what mechanism would that take place, but I'm going with "highly unlikely" on that one
Top right:
> About > SOURCE CODE for "Database Performance at Scale: A Practical Guide (Apress, 2023)," by Felipe Cardeneti Mendes, Piotr Sarna, Pavel Emelyanov & Cynthia Dunlop
But readme.md:
> This repository ACCOMPANIES Database Performance at Scale:A Practical Guide by Felipe Cardeneti Mendes, Piotr Sarna, Pavel Emelyanov & Cynthia Dunlop (Apress, 2023).
[ my emphasis ]
For comparison, https://doc.rust-lang.org/book/ is an open source book. A PDF with a CC license without a repo of the publishing artifacts is not an open source book. It's just a free book.