No. That is incidental. I am arguing from basic concepts related to intelligence. Note how I can argue from the same concepts regarding dogs and chimps. Not just humans. I am arguing what i think will be likely.
That is because you are also anthropomorphizing dogs and chimps. There is a difference between eliciting a trained response and real two-way communication of complex ideas. You don't get the same level of understanding from the wild dog or monkey- to him, your smile is a threat. Or he may even see you as food..
"They would understand us as sentient and aspossessing a language with syntax, abstract concepts, etc. I believe this would almost certainly be true, because I believe they would also necessarily possess language."
These are all things
you possess. They are human attributes.
Dolphins are mammals, very similar to us. We can train them to do tricks, and we know they communicate with each other. We consider them intelligent. But we cannot know their thoughts and we can't decode their sounds.
When they surf the bow wake, they look to me like they are having fun. But can I say that's why they do it? Maybe they are just being lazy and hitching a ride. Or maybe they like the boat because the sharks don't. Or maybe they are just hungry and they think there might be something to eat.
I can't attribute their behavior to a human desire or fear or emotion- they are not human. They can have motivations that I have no hope of comprehending.
Compared to the aliens, dolphins are practically our twins.
Your aliens seem to me like the Star Trek aliens. Basically funny-looking people. I don't deny the possibility, but I don't give it more weight than any of the other infinite possibilities, like the flying ammonia jellyfish.
If I met an alien would I be scared? Hell yes! I have no idea what I am dealing with.