There Is literally no difference. Is she assuming import of other brassicas? Because that ain't it."Softer on the environment that their brethren"???? How is a cabbage any "softer on the environment" than a cauliflower, or sprouts, or broccoli or turnips (yes they are brassicas) or sweeds or...oh you get my drift.
That is before you include non brassica UK grown vegetables such as runner beans, carrots, leeks, etc etc.