As another commenter pointed out, the 1.5Tbit/in^2 number in the posting (which is taken from the original article) is pretty bogus. Seagate's 2TB 7mm 2.5" drive has an areal density of 1.32Tbit/in^2, and it's probably a safe bet that they (and WD) can wring another 15% density improvement out of SMR technology in the next year or two.
For those commenters bemoaning the fact that the highest density drives today are SMR rather than "regular" drives, get over it - the odds of conventional non-HAMR, non-shingled drives getting much denser than the roughly 1TByte per 3.5" platter we see today are slim to none:
To get smaller bits, you need a smaller write head. That smaller write head has a weaker magnetic field. The weaker field means the media has to be more easily magnetizable (i.e. has lower coercivity). The lower coercivity media needs to have a bigger grain size (size of the individual magnetic domains), so that grains don't flip polarity too often due to thermal noise.
Since a bit can't be smaller than a grain, that means that smaller your write head is, the larger your minimum bit size is. Eventually those two lines cross on the graph, and it's game over.
Two ways of getting out of this are SMR (shingled magnetic recording) and HAMR (heat-assisted magnetic recording):
SMR - stop making the write head smaller, but keep making the bits smaller. Overlap tracks like clapboards on the side of the house (where'd this "shingle" nonsense come from?), allowing small bits with large write heads. Of course this means that you can't re-write anything without wiping out adjacent tracks, which means you need something like a flash translation layer inside the drive, and because of that, random writes might be *really* slow sometimes. (I've seem peak delays of 4 seconds when we're really trying to make them behave badly)
HAMR - Write your bits on low-coercivity media with a tiny, wimpy head, and store them on high-coercivity media with tiny magnetic grains. How do you do this? By heating heating a high-coercivity media with a laser (say to 450C or so) to reduce its coercivity to reasonable levels, then letting it cool down afterwards. But you need a big laser (20mw?) on each head, which causes a whole bunch of problems. Which is probably why they're delaying them.
Oh, and you can overlap tracks on HAMR drives, creating an SMR HAMR drive, with even higher density but the performance problems of both technologies. Which they'll probably do as soon as HAMR hits the market, because with today's SSDs the market for fast HDDs is dying a very quick death.