I think its been conclusively demonstrated that deep learning can discover correlations that go unnoticed by humans. An example that immediately comes to mind is AI detecting race from X-rays. There are many others. There's also many other factors that influence our ability to ascribe language skills to other groups of humans, from complexity of vocalizations to similar ways to interact with the environment and with other group members. We recognize ourselves in fellow humans, which makes it easy to ascribe similar capacities. The question is how easily can we recognize language in a species where we do not immediately recognize ourselves? The more foreign the behavior and the patterns of communication, the harder it gets. AI has the ability to elide over those difficulties.

I don't think it's implausible. I think it's more implausible that human language is unique

I expect there's a hierarchy in language acquisition, where describing interactions with the world come before more abstract talk such as gossip. But once this first-order descriptive talk is understood, this can be leveraged to capture the more abstract talk.

I think so. It's similar to how an adult can learn a new language by way of immersion. Correlations and gesturing go a long way.