Absolutely, completely not true at all.
The 4S was always the plan.
I agree with this a million times over.
At some year's D8 conference, Steve said that when Apple releases an iPhone, "we're working on the next one, the one after that, and thinking about the one after that."
That means that the 4S would have been in the pipeline as early as around the iPhone 3G and in development at the time of the iPhone 3GS. It wasn't a "last-minute decision" to fill in for a delayed "iPhone 5."