Sorry, but you are missing the point. Intelligence is an incredibly human-centric concept. Everything we consider “intelligent” behavior, or “intelligent” traits is relative to humans. A good example is how we consider Chimpanzees a “remarkably intelligent species” because they use tools like we humans do. However an ant colony is not considered intelligent even though it has large scale agriculture, social hierarchy and communication techniques we don’t fully understand. And let alone fungal networks which behavior is so alien to us that it could be exhibiting behavior very intelligent to them, but our understanding of fungal life is so limited that we can’t even begin to see it as ‘intelligent’.

If we do come across extra-terrestrial life (more likely through our telescopes, rather then by paying a visit) it will be even more alien to us then our mushroom cousins. It will have evolved in a planet vastly different from us, it might communicate in a way we can’t even think of. Our technology will have more in common with the ant colonies then with the alien technology.

If we do come across extra-terrestrial life and are able to marvel at their intelligence, then I think it will have more to do with our ability to understand things out of our scope, rather then some absolute and arbitrary metric of intelligence.

> However an ant colony is not considered intelligent even though it has large scale agriculture, social hierarchy and communication techniques we don’t fully understand.

Actually, ants are frequently deemed smart and intelligent. Such suppositions are common.

Indeed, and there are also people actively investigating plant intelligence.

The question that arises though, when we find more and more examples of intelligent behavior across the tree of of life, is intelligence actually that useful of a concept?

Depends on your definition of intelligence. But even if more intelligence means a higher chance of finding others first, there's still good old luck. Especially the luck of being the first born:

Humans have been roughly at their current biological level of intelligence for at least thousands of years. But our ability to detect aliens has increased dramatically. Similar, a slightly more intelligence alien species that's lagging us by a few million years would have a hard time detecting us anytime.

Or, conceivably dolphins and wales might be more intelligent than us. Or you could conceive of a highly intelligent squid like species on another world. Problem is, if their whole world is an ocean, they are going to have a harder time with metallurgy or electricity.

(It's probably not impossible to build up technology under water, but it's probably harder and requires more cleverness and time. And even if it doesn't, because there's an easy way that humans just don't see; there are probably some circumstances that make it much harder. Eg assume a world that's all ice or desert, so it can only sustain a million people instead of billions.)

“For instance, on the planet Earth, man had always assumed that he was more intelligent than dolphins because he had achieved so much—the wheel, New York, wars and so on—whilst all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man—for precisely the same reasons.”

― Douglas Adams, The Hitchhiker's Guide to the Galaxy

Agreed, and by some interesting heuristics, we already know animals significantly smarter then us. Watching the chimps absolutely dominate humans in memory matching games is fascinating.