Of course, I don't expect it to!

But in the past 6 years of your career, I expect you've studied those things a lot more than I have. Do you think that the book is still a good starting point for someone looking to progress beyond dumping confusing vendor binaries in off-the-shelf Bluetooth modules that seem to be energy hogs, to miniaturize and integrate custom firmware and onboard antennas on my PCBs?

yes, I highly recommend Heydon's book, even in 2023. The tech stack including l2cap, connection establishment, advertising, connection parameters, security, GAP, GATT, are all still relevant.