" the foam on bitter is made up of air bubbles, the foam on a pint of Guinness is made up of nitrogen"

No - bitter is made up of CO2 bubbles (as appears to be acknowledged later in the paragraph) except for "smoothflow"-type keg bitter which uses the same 30/70 gas (30% CO2, 70% nitrogen) as Guinness. There's no separate nitrogen cylinder, it comes premixed as 30/70 - using pure nitrogen would make it come out as solid as whipped cream.