On her Majesty Secret Service was a reboot?

Indeed. I understand why they took this approach, especially as YOLT’s poster proclaimed ‘Sean Connery IS James Bond’. Delaying Bond’s introduction in the pre-title sequence built up the supporting characters such as M, Q and Moneypenny, and we see the Aston Martin before Lazenby himself is fully identified. As you say, the intention was to make the world feel like an authentic continuation.