Well, I bought and sowed seed of Ailsa Craig onions this year, they came from somewhere!

All onions will set seed if allowed to grow long enough. Possibly what you were told is that Alisa Craig do not normally 'go to seed', in other words they don't 'bolt'. Left long enough they will flower and set seed though as will almost all plants.

Take brassicas as an example and oil seed rape specifically, it is sown one year and does not flower and seed until the following year. This is true of all leafy vegetables that we grow.