My Cow Plants have never produced seeds themselves. I have never seen a Harvest option in the menu when I click on them. But the spliced Snapdragon/Dragon Fruit plant does produce Cow Plant Berries, along with the other two items. When they tweaked the rareness of some produce spawning, it may have affected how many. That must be what they mean? It does sound like the actual "Cow Plant that will produce seeds..." from the text, tho, doesn't it?
If you haven't noticed, you can take those Cow Plant Berries, plant them, and as soon as it grows into a little "Calf" Plant, you can sell it for several hundred simoleons. It's a nice boost if you're poor at the time.
I haven't pursued getting Cow Plants as much as I used to before Cats and Dogs, so it is possible I just never ran across this feature.