Investing more at the seed stage also makes sense because it allows the investor to diversify with a smaller amount of capital, and potentially exit earlier (if the company can get late stage funding and the early investors can get out through it or a secondary market).
This behavior is in line with your observations of a concentration of investment in winners (and in line with GSV Capital's public statements from the 2Q earnings call earlier this week). It is also consistent with the article's statement that now may be a good time to quit your job, as seed stage is frothy.
It's not clear that's what is occurring now. I think you're confused by the outsized deals reference about last Summer - they're not saying that's what is causing the index to spike now; they're saying it's what caused the spike last Summer, such that comparing today vs a year ago should come with a flag about that data.
It's funny you mention that ... over the past week, I've found myself seeing more bloomberg promoted posts on facebook (I've been purging/blocking "viral" pages from my feed), and thought to myself, "these are awfully clickbaity". This is anecdotal of course ... but thought it was funny to see the same thought mentioned here :)
So it's a concentration of investment in likely winners. Unless you have some unique edge (i.e. you're B2B in an untapped niche with a near-guaranteed path to profitability and an extensive client list waiting in the wings) it might not be the best time to quit your day job.
And I expect better from Bloomberg when it comes to clickbait.