It isn't an official phrase, but what I mean is adding up ELO ratings for each year. So for instance, let's say one player has five years at 2000 ELO, and another four years at 2400. The former adds up to more (10,000 vs 9,600), but most would think the latter was a better player because 2400 is truly great, while 2000 is merely garden variety elite - roughly a top 10 player (e.g. Casper Ruud has a 2,012 ELO right now, 10th in the ATP).
So I'm guessing his formula adds up year end ELO then averages it with a player's best five or seven or ten seasons, or perhaps a more complex version of that.
BTW, UltimateTennisStatistics.com has ELO ratings - total weeks at #1, year-end ELO for every player, etc. Kind of fun to look at, as an alternate window into rankings.
Here's a chart with the year-end Elo Ratings of Nishikori, Wawrinka, and Ferrer.
View attachment 7260
Based on that, it is hard to see why Kei is ranked higher than Stan. Ferrer actually has the highest peak, but both he and Stan have significantly longer periods in the elite range, and Stan's peak is as high as Kei's. I'm actually a bit surprised Stan's isn't higher. I'm guessing that despite those three Slams, his overall performance in those years wasn't consistent enough for a higher rating. And when you add in Stan's three Slams (and one Masters), it is hard to see how he could be ranked lower - especially than Nishikori.