How about the part where the team scaled up to a dozen people? Do you disbelieve there was enough work achieving feature parity to keep a team of engineers busy?
The article doesn't report it took JK 18 months to be able to boot into OS X, simply that that's when it got greater visibility. I think it's highly plausible that 1.5 years was enough time to get somewhere between "won't boot" and "everything works great".
Edit: re-read, and realized I made an assumption that wasn't explicit. I'm assuming that OS X diverged in significant ways from Next, and did not maintain compatibility for non-PPC architectures. I think that's highly likely. I'd also wonder how much of JK's time was spent dealing with device drivers.
The article doesn't report it took JK 18 months to be able to boot into OS X, simply that that's when it got greater visibility. I think it's highly plausible that 1.5 years was enough time to get somewhere between "won't boot" and "everything works great".
Edit: re-read, and realized I made an assumption that wasn't explicit. I'm assuming that OS X diverged in significant ways from Next, and did not maintain compatibility for non-PPC architectures. I think that's highly likely. I'd also wonder how much of JK's time was spent dealing with device drivers.