Greg Nuckols wrote that in a Stronger by Science article.
I’ve been playing with this myself over the past 6 or so months, the idea of counting hard sets rather than adding up sets x reps or sets x reps x weight.
The reason I’ve moved to it is simple: if most of the methods are flawed, and all seem to yield very similar results for muscle growth, then choose the easiest simplest way. For me, that means counting hard sets. There is ambiguity in what determines “hard”, but there is also ambiguity in the other methods as well (i.e. at what point in the warm-up sets for volume load do you start counting that?)
So, if you’re training hard, try counting up the number of hard sets you’re doing each week for a given exercise or group of muscles. If you haven’t seen any growth, are you doing too little? Are you pushing hard enough? Are you eating to support growth?