Not as horrible as you might think, but pretty horrible. If this is true, what will vegans and vegetarians eat? And if you compost a vegan (composting remains is quite the craze these days), do plants not use them for food? - or since they ate plants only, would the plants not like eating them better - or be more satisfied, exacting some degree of revenge?
Is it a documentary or an extremely dry mockumentary? It had a kind of tongue-in-cheek tone to it, splice with plant-trauma.
On Stevie Wonder's contribution -
First of all, I love Stevie. An amazing talent. His appearances were often a surprise. It has been a long time (like, "midnight movie when it first came out", long time) but it was nearly comic relief when he showed up.
I'm sure that was not the intention.
Looking back, The Attack of Killer Tomatoes may have been the perfect sequel whether they knew it or not. It is "revenge of the plants", essentially.