I'm not sure what you're confused about really? The F progression would be 1-b4 -5, to me anyway! I'm not a music theorists but I think I got this one. Just as if that were in the key of G.....then 1-4-5 (G, C, D).......it's the same in F except it's Bb with F so it's a b4..........at least the way I interpret it...... but what do I know??
It's a great song and you know the progression....at least on the instrument.....so why fuss over it on paper??