Coding for a PowerPC is completely different from coding for x86 (the instruction set that Intel and AMD processors use.) Since most exploits are released for x86 platforms, it means the coders are familiar with x86 coding, and are therefore now familiar with the CPU-related coding for Apple computers.
However, it's just alarmism, since exploits run on Windows, not x86. OS X isn't Windows.
I understand that the machine language instruction sets, the methods of accessing memory, etc are different. But I was curious if there was actually something about it that made it more susceptible to exploits. The only think I can think of is that the vast majority of the buffer-overflow type of exploits have been geared towards the x86 instruction set, partly because Windows allows for these exploits and partly because it is simply so prevalent. So, even though you wouldn't be able to rely on the various exploitable DLLs in OS X, you might still have a bunch of machine language code that had been tested on the hardware already.