The open source NeXTSTEP underpinnings of OSX definitely helped. Even before Apple took over and turned it into OSX it was already running on Motorola 68K, RISC and most importantly Intel x86 CPUs, so Apple already had a head start in the x86 code development. Plus, as you say, Apple is a huge company (now second only to Microsoft by a small margin in terms of cash wealth and the success of the iPod and iPhone).
And it makes perfect sense to the rest of the world to make Amiga OS open source. If anyone was being realistic about the OS, it is in no way mainstream. How many copies of Amiga OS 4 and 4.1 have been sold? That number is limited to the number of working PPC powered Amigas still in operation at the most, which can't be that many. Hyperion therefore have a very finite marketplace to sell their OS to, although I would say they can guarantee that all current PPC Amiga users are likely to purchase a copy of each new version, so they do at least have that. Although that is always being to be a very limited market with no room for real expansion or commercial success.
However, hasn't the ability to port Amiga OS to other platforms been held up until now by the on going dispute between Amiga Inc and Hyperion? In the settlement statement they stated:
"Hyperion is granted an exclusive, perpetual, worldwide right to AmigaOS 3.1 in order to use, develop, modify, commercialize, distribute and market AmigaOS 4.x (and subsequent versions of AmigaOS including without limitation AmigaOS 5) in any form, on any medium and for any current or future hardware platform under the exclusive trademark “AmigaOS” (Amiga operating system) and using other associated trademarks (such as the “BoingBall” logo)."
For me, the crucial wording in that statement is "in any form, on any medium and for any current or future hardware platform". This would indicate the license now grants them the right to port the OS over to other hardware platforms, which sounds like they were not able to do before.