You're right that that is distinct from “hours worked”, and sure, maybe 35 is the magic number for someone.
0 hours is too few hours, and 70 is too many— so clearly there is a line there somewhere. I suspect that that line varies from person to person and even week to week, hence the point about “either be working or don’t be working”.
Similarly I've been in and seen hard working teams make chains of silly bad decisions that had to be corrected with even more work later. So I guess keep in mind the corollary is that it’s a marathon not a sprint. And maybe so it’s less of a trope: there is “too long” on both a low level everyday as well as a larger scale - come up for air and breathe and get your bearings and perspective.