I wrote something along those lines, then replaced it with what I did post, just for brevity. You said it better than I would have.
Mutualism is a well-proven successful strategy. There does seem more scope for intelligence in crafting positive-sum arrangements than in simply competing. (Arguably our own brains hypertrophied in response to sexual selection of better storytellers, with other results secondary.)
usually, we just presume the strategies were stored in genetic level.
We can presume, but Nature keeps her own counsel. Anywhere there is any scope for intelligent behavior and capacity to exercise it, we should expect to see nature explore that, sooner or later. I know no reason to think that could not have been done yet.
There is some scope for winged humans, and hypothetically some capacity to exercise it. Nature did not explore this route. The opportunity cost of exploratory settling into a non-preferred niche usually outweighs the benefits of that exploration simply due to the scarcity of available resources to buffer against the opportunity cost.
But, as noted, for this case there is very little in the way of cost.
Nature did, in fact, explore the winged-bipeds body plan to full fruition twice, once in mammals, and did winged quadrupeds before and again in mammals. She is progressing on a another as I write this. And, humans did end up conquering the air without growing wings. Nature isn't picky about methods.
> trees can afford to be very, very smart.
Hopefully I can use some of my perspective to address your follow-on question:
> What could a smarter tree do to outcompete rivals?
This may represent a limited understanding of Darwinian evolution.
Trees have an economy, sharing resources with each other and with other living things.
Tree also are (more or less) fixed in place, thus these economic games are repeated games with known participants.
They thus have a vested interest in maximizing the wealth of the community, rather than their individual wealth. They may (and studies suggest they do) have a preference for supporting their offspring over other trees, but not to the point of killing the community their offspring will also depend on.
Where I am going with this. Repeated economic games are not zero-sum; the pie can get bigger. "Competition" in this setting does not look like competition in a zero-sum "I have to beat the other guy" game.
What trees can do with their intelligence is learn how to build long-term stable and nuturing communities which generate substantial wealth for many members. And that this is a logical local (perhaps global) optimium.