I e-mailed Krista @ stumptuous and this is her reply:
It's true and not true. It's true in the sense that long duration low intensity cardio (aka endurance cardio) does place the body into a catabolic state. It's not true in the sense that this doesn't magically happen after one hour (so that at 65 min you're exactly 5 min into burning muscle). Basically, endurance work if done repeatedly over the long term does contribute to a physical state where the trainee is less likely to retain muscle mass. This is one reason why marathoners look the way they do (and why marathoners and sprinters look so different). It's also one reason why I recommend that most folks opt for interval type cardio training if they are not training to be endurance athletes.
However, one has to consider the total training load. If one does one hour of endurance work five times a week, the catabolic effect will be much more significant than someone who does one hour once a week.
Hope that helps.
So I'm afraid Meg and Mel, she doesn't offer musch science behind it, but I'm please that I can stop worrying about burning muscle during my double kickboxing sessions!
Darn evolution's to blame for everything!
