@Xeramtheum I also find it to be a very odd way to describe someone's gardens! Personally, I would take it as a compliment, because, as mentioned above, nursery plants are healthy and lush!
But, beauty is in the eye of the beholder, so perhaps it was a compliment, or perhaps not!
As mentioned above, it may have been described that way because your friend has a great deal of pots, but I'm going to take that a step further. There are decorative pots that are black, right? So, perhaps your friend has a large collection of decorative black pots? .....and/or their plants, whether potted or in the ground, are lined up like soldiers, just as they are displayed at nurseries?
I hope your friend is able to get past this, and be happy with how THEY view their own gardens, because that's most important to their own inner peace!!