I thank you for finally citing a source, but by his own admission Riggs is relying entirely on hearsay, and not at all on any actual sales figures, in making that claim.
Quoting from the article: "
It's the only edition I have no numbers for. But every creator told me that that it sold worse than 3.5. And 3.5 sold worse than 3.0. And 3.0 sold worse than Second [Edition]. Second [Edition] sold worse than First [Edition] and there we are."
I hope that if Riggs does plan to publish a 3e/4e follow-up to
Slaying the Dragon he is diligent enough to consider the digital sales of 4e products in any comparisons he does.
For the first nine months of 4E, WotC sold PDF versions of its books—everything up to and including the
Player's Handbook 2—concurrently with the printed books
. This practice stopped abruptly after the PHB2 in a knee-jerk reaction to piracy, but a fair comparison of sales across editions would, I hope, include those digital sales.
Then, we need to consider D&D Insider. That didn't have anywhere near the volumes of users that D&D Beyond has now, but for a monthly subscription fee of US$5.95, you could buy/rent access to
all of the 4E content, so it was fairly popular for 4E fans. An
analysis of the number of D&DI subscribers put the
minimum at 80k in mid 2013. Compared to Riggs' sales estimate of 300k total 3.5 PHBs, that is a significant number.
Given this, unless we are agreeing to measure how well an edition sold based purely on the sales of printed books, 4E's digital sales seem relevant for comparison purposes.
Anecdotally, not one of the players in my 4E campaign bought a single printed book. But most of them paid for D&D Insider accounts. (The others borrowed the accounts of the ones who were paying.)