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.
I also have my doubts on the 78% figure given by Yahoo Finance. Would like to see the methodology they used to reach this figure. If they classified all the coins that haven't moved during the last 12 months as those with no intend to sell, then even my coins would fall under that category. But it is simply not true. I have intention to sell and I will do it at the right time. Just because I haven't moved my coins that doesn't mean that I am going to hold on to them for the next 20 years.