That happened in 2006, and I would argue that the effect of them moving from PowerPC to Intel was dwarfed by the effect of people wanting to be able to do things like iPhone app development.

I think it was a bit of that, mixed with the iPhone launch and subsequent release of the App Store and third-party apps. Limiting app development and distribution exclusively to Macs through Xcode probably forced a lot of people into using them for the first time. Mac OS X also received UNIX certification around this time IIRC, I wouldn't be surprised if that compatibility pushed some more people tired of fiddling with their OS towards the "it just works" Mac.

Macbook-as-developer-machine started getting popular around the release of the iPhone. Apple took the opportunity to lock down their ecosystem and allow you to exclusively build and sign applications on their own platform, basically locking out anyone who isn't a Mac owner.

I'm not understanding what you're contrasting when you say they locked out people who didn't own Macs.

From 1984 to the iPhone, I believe it was the norm to develop Mac applications on Macs, except for a brief period when the very first models were starved for memory.

I wonder how did Mac software happened before, given that apparently there were no developers.