Eh, they had inside information (the Earth-Trisolaran Organisation was in contact with them).
And it's hard sci-fi because they were AIs made of protons, not actual fairies found in the magical forest of Elthrolien that had to be seduced with promises of space mead.
That's how it goes with sci-fi, innit. You can come up with anything you like as long as it's obvious that it's just advanced technology, not magic. Cixin Liu usually manages to weave in a couple of natural laws to every impossible thing so he passes.
I was more annoyed by the lightspeed contrails to be honest. That really comes out of nowhere and is a total literary device that has no basis on anything we know of. Makes the whole endeavour space opera if you ask me - which is not bad in and of itself. But in that case, where's the nuclear energy-sword wielding hero who saves humanity? Disappoint.