A bubble is all-pervasive and extreme. It represents a systematic investment mania where everything becomes surreal. People sell vast tracts of land for a prized tulip. Junk companies with nothing to offer but a vague concept about revolutionizing how this or that will be done owing to some new phenomenon such as the internet make serial stock offerings to the public and get hundreds of millions for a modest percent of their unproven company. Lenders pile on with countless real estate loans to unqualified borrowers secure in the belief that what are really worthless loans will make them huge profits because they can be packaged and disposed of through artificial securitized instruments and because housing prices will continue rise broadly for endless periods. All this begins to occur in endless and ever-expanding streams until, in the end, large numbers of people are sucked into the vortex.
In such cases, broad markets affecting an entire society are sent into a frenzy by which average people start both to get rich quick and to want to get rich quick. Large numbers of people leap in, therefore, in the hope of making fast money and abandon their common sense in the process. And when things go bust, this has a major systemic effect on the broader economy. A stock market that had reached stratospheric heights loses 70% of its value. A real estate market that had become so pricey as to make housing unaffordable for average buyers plummets to the depths, taking down people's savings en masse.
The current phenomenon represented by high valuations in parts of the startup world is more transient and limited. It has not affected the broader society at all, only an insular investment community. If it fell apart today in toto, it would leave a trail of victims within the VC and angel communities but would be felt scarcely at all in the broader economy, or at least would likely have no systemic impact.
Viewed from the standpoint of the broader society, I think what we are looking at here is a speculative frenzy affecting a comparatively narrow asset class. The prices of some startups have increased considerably. The prices of companies generally in the business world remain moderate, if not depressed. Is it a pricing frenzy within a particular segment of an asset class? Probably. Is it a bubble? No. Or at least not by historic definitions.
Again, I wouldn't disagree with a single specific point made in this piece, and the author as usual makes some astute observations. I would disagree about the terminology, though, and would say that we should reserve use of the term "bubble" for the sorts of massively dislocating events that it historically has come to represent.