Since the block reward halves every 210,000 blocks (~4 years) half of all Bitcoins that will ever be mined were mined by around the start of 2013, at which point the price was as low as $15.  It's no wonder, therefore, that a large proportion of Bitcoins are either lost or not in circulation.  Where this becomes a problem is if just a small percentage of these early miners begin to unload coins, the price will fall significantly.  I expect that many of them, rather than having coins which will just never enter circulation as the article implies, are biding their time and selling a bit at a time, which will increase the supply alongside the million or so new coins that will also enter the market before the next halving.

I definitely disagree with the 78% figure because a significant amount of BTC is being held by people who are waiting for ideal times and by exchange cold wallets.