They prove no such thing. We can't even prove consciousness in other humans.
I’ve kind of thought this for many years though. A bacterium and a tree are probably conscious. I think it’s a property of life rather than brains. Our brains are conscious because they are alive. They are also intelligent.
The consciousness of a bacterium or a tree might be radically unlike ours. It might not have a sense of self in the same way we do, or experience time the same way, but it probably has some form of experience of existing.
How is that different than a cell?
I don't think it's that unusual. It seems to me just to be a narrower version of panpsychism:
An animal that doesn't have some kind of pair bond or social arrangement, and doesn't raise its young, has a lot less need for some of this emotional hardware than we do.
Whereas K-selected species that raise their kids have broadly the same need for it as humans.
That doesn't categorically mean it evolved with the first pair-bonding K-reproducer, or that birds have parallel-evolved emotional hardware like ours, but there's plenty of behavioural evidence there - the last common ancestor of birds and humans was small-brained and primitive, but investing in individual children probably evolved around the time of amniote eggs, just because they were so much more biologically expensive to produce than amphibian or fish eggs.
Trees react to the world around them in many ways.
If a single cell organism moves towards light and away from a rock, we say it’s aware. When a roomba vacuum does the same we try to create alternate explanations. Why? Based on the criteria applied to one it’s aware. If there is some other criteria, say we find out the roomba doesn’t sense the wall but has a map of the room and is using GPS and a programmed route, then the criteria of “no fixed programs that relate to data outside of the system, would justify saying the roomba isn’t “aware”.