This model isn’t based on release planning at Guinness. It’s based on drinking Guinness.
It’s the end of a work day. You and your mates are going out for a drink. Initially, you’re thinking you’d like to have three pints of Guinness. How do you place your order?
If you’re a traditionally-minded software development project manager, you’ll order all three pints at once, in the same glass. When the bartender tells you three pints of Guinness won’t fit in a one-pint glass, you’ll pound your fist on the bar and shout until he finds a way to make them fit. After all, the reason he doesn’t want to pour three pints into the glass is that he’s either lazy or incompetent, or both. Once you let him know who’s boss in no uncertain terms, he’ll get those three pints into the glass, one way or another.