I find it very hard to rationalize the information. I fail to see why Sony would add so much RAM, and a relatively expensive type at that to simply add 'snappier' multi-tasking or multi-tasking with numerous apps at the last minute, especially when Cerny said he added it because of the request of a game developer. From a decision making standpoint, it doesn't make sense to add so much GDDR5 ram due to 'listening to developers', only to reserve the vast majority of that for the OS and to give developers perhaps an extra gig to play with.
Sony is betting on the PS4 being a 'games-first' machine this time around and GDDR5 RAM is a significant expense. I doubt Sony would be stupid to incur all of that extra cost for multi-tasking or some OS feature they cooked up over the period of a few months. The average gamer simply doesn't care about that, so the cost to benefit ratio wouldn't be there. The One was designed from the get-go to be a multi-feature box. Sony announced the ram upgrade PRIOR to all of the information being released on the One, so I don't see how they could have doubled the RAM in reaction to obtain OS feature parity with the One.
The video sharing was built into the system before the addition of the extra 4GB of ram, so that being the cause of this speculated OS RAM bloat is out of the question. There is nothing you could possibly add to the system over the period of months that would require the OS to reserve possibly 7 times more RAM. For one, Sony has never been one to build these overly elaborate OS's. Doubling the RAM for OS features you don't even have planned doesn't seem like reasonable decision making to me.
The article says 4.5 gigs of RAM is available for use and that an additional GB 'may' be reclaimed in the form of flex RAM. How is that a 'maybe' when Jonathan Blow already said he was using 5 GB for his game? That completely throws that figure on its head. Also, with the tone Jonathan Blow was using, aka he is 'already using 5 GB for his game' gives me the impression that he has a lot more RAM to still play with. If he was limited to 4.5 GB and a possible extra 1 of flex RAM, he'd be complaining about the PS4 being RAM constrained and being forced to tap into this flex RAM. This issue could easily be resolved by asking Jonathan Blow if he had to use .5 of this flex RAM for his game.