I think the writer of this article has missed one very key point. Microsoft concern about GDDR5 wasn't availability, but the memory limit it put on the system at the time.
In a device like this, unless you're going to use riser cards, the number of RAM modules you can put in is about 16, which both the PS4 and Xbox One have.
At the time the system were being designed the largest module available for GDDR5 was 2Gb, giving a maximum system memory of 4GB (2*16/8).
Microsoft needed 8GB to do what they wanted, Sony were happy with 4GB.
This meant they had to use DDR3, which in turn forced them to use die space for the ESRAM.
Then in early 2013 4Gb GDDR5 modules became available. Moving from 2Gb to 4Gb is an easy change, so Sony did so. For Microsoft to change at this point would have required throwing everything away and starting again, not really an option.
In other words Microsoft's 'mistake' was to want a system with 8GB of RAM.