It’s possible the fence originally housed something else and thus the cabbage remains safe?I'm surprised they've left it to grow and flower.
It's an invasive weed.....I think it's technically an Invasive Alien Species.
Could be they didn't know what it is but it looked interesting/unusual & so protected it. I have done similar in the past when I came across a stunning unknown plant in the garden & then got all excited thinking I had something special. They invariably turned out to be common or garden weeds that I hadn't seen before.I'm surprised they've left it to grow and flower.
It's an invasive weed.....I think it's technically an Invasive Alien Species.