It seems like this assumes some kind of conservation of failure

Yeah, more or less. From my personal experience, it seems to require about the same amount of willpower to get either a string of small failures or a single big failure. I have no clue why this is, beyond the basic theory of "success chains" being good for motivating us - a single break doesn't seem to slow down motivation, but a lot of little ones tend to kill it.

Hmmm, given that some people look at this advice as "obvious" and others are utterly baffled by it, there's a chance that this advice only works for a certain segment of the population. It might help to model this as general advice, regardless of goal: I learned about it in terms of building career skills and fixing sleep schedules, and just naively started using it to build my diets on the assumption that it was a generic pattern (for me, at least, it's where all my semi-stable diets come from)