When I look at the context of that statement, it is in the 3rd of 7 paragraphs talking about the pitfalls of using TSS, PMC, etc. The comment is based on the fact TrainingPeaks uses HR to estimate TSS in the gym. We could say the same about using HR for TSS in cycling, it can be hit or miss.
TSS was defined as a power based metric, taking into account both intensity and duration. Would you ignore the power meter on your bike, and use HR for TSS just because TrainingPeaks allows it?
The question to ask – why does TrainingPeaks ignore intensity/duration when calculating TSS in the gym? The whole point of TSS is to take intensity and duration into account, and in the gym this is sets, reps, and weight as % of 1RM. Having strength training TSS based on HR is just adding a feature to claim support, despite the fact its somewhat arbitrary and perhaps meaningless support of strength training.
“Intensity Number of Lifts” (INOL) is taking the TSS concept of wrapping intensity/duration into a number, and applying it to weight lifting.