I don't think it's as obvious as you make it out to be. For one, what we consider evidence of "thinking" is autonomous action by a being as a whole. For as far as I know, there is no evidence that a plant's movements are governed by a central coordinating authority. Can your evidence distinguish between a plant having a single mind or each branch having decision authority of its own?
Why does a plant’s movements need to be governed by a “central, coordinating authority”? Isn’t that employing another anthropomorphism onto a plant?
Octopi have, what, 9 minds? Would we consider them a single or multiple creatures?
And to think of plants, some single organisms span *acres* of connected root structures.
So, why would one need to distinguish between a single mind and multiple minds in one creature? To what end does it serve, except managing a narrow, specific definition of thought and/or memory that doesn’t even model the whole of creatures we generally agree are living and thinking?
I’m not saying I know if they’re a “self-replicating, autonomous food source” or not; but I am saying it’s quite presumptuous to declare they’re not and then to gradually narrow the definitions when a plant surpasses some line in the sand we didn’t think it would pass.
It reminds me of all the human exceptionalism stances we had over the centuries, “humans are special because they use tools” and the like.