If it were a "simple" card swap out, I could see that as a possibility.
It's much better than the "we're still waiting because of heat issues" theory. I say this mainly because they've had chips in hand (for testing and R&D) for what now, over three months? If Apple's engineers couldn't have solved it in that time, they need new engineers. The only possibility is a shift in the release timeline, which could have moved it to the right a tick.
One possibility might be that the supply of chips still isn't up to speed enough to launch the new line(s). Last I read they had only shipped 5 million chips total so far about a week or so ago, and that was between all manufacturers and not just for Apple. If the supply availability hasn't come up to speed yet, it could be a possible reason for the trigger not being pulled yet.
At this point, it's anybody's guess.