if you test the human with any known limitation of their architecture (like making conclusions or just changing the form of the question enough) you will immediately see that is in fact very much not even remotely close to sentient
But it does now follow, and it is by no means certain that such life would be sentient in any way recognizable to us, or us to them.
You're doing what everyone else is doing - conflating a link between intelligence and sentience that's just a projection of your human bias.
The two movies don't do justice to that theme, or at best they do it only in the most glancing of ways before rushing back to a standard deus-ex-machina in the end. In each case, the film makers seem to have lost their nerve, as if the idea of presenting that enigma to the audience in a more direct and accessible way is just too hard of a problem.
But for me personally, it's on the short list of science fiction works that still stick with me in a personal way, and that I like to return to every few years. And, yes, it is an arrogant viewpoint to say that all we ever do as humans is look for mirrors of ourselves. But I think Lem got at a pretty deep truth about the human condition when he did that.
maybe you can give a fitting example of an English sentence that these aliens would come up with, which humans would be totally unable to respond to in a way which makes sense?