In the ending of the story "A Very Old Man with Enormous Wings" by Gabriel Garcia Marquez, it says that the angel is an imaginary dot on the horizon of the sea. So does that mean that the angel is actually imaginary?

The nature of the angel in the story is ambiguous. By referring to the angel as "an imaginary dot," the author could be suggesting that the angel was a figment of the imagination. However, the author could also mean that Elisenda keeps watching the angel flapping away until she can no longer see him, not even the faintest trace of him, but she imagines that she does.
The story is written in the style of magical realism. This literary style combines realistic and surreal elements. The angel is presented in a kind of matter-of-fact, even mundane way in the course of the story. In fact, he has ragged wings that are missing half of their feathers, and he seems far from being an angel of myth. He instead resembles a broken-down old man. However, when he takes flight, the reader, and Elisenda, have to wonder whether he really is an angel. The story asks the reader to remain mystified by this creature and to remain wondering whether he is real or a figment of the imagination.

Great question!
 
No. It doesn't mean that. That's an option, and a possibility, but not a certainty. First, let us review the final line of the story. Marquez wrote, "She kept watching him even when she was through cutting the onions and she kept on watching until it was no longer possible for her to see him, because then he was no longer an annoyance in her life but an imaginary dot on the horizon of the sea."
That's a crucial distinction. Marquez doesn't say the old man is an imaginary dot per se. He's saying that's how Elisenda sees him. This is a narration of Elisenda's view of things. She's so fixed on watching the old man that she keeps staring even after she can't see him. The most likely meaning of the "imaginary dot" is that she stares so hard she sees little shadows or birds or even flecks in her eye as the angel. She wants to keep seeing him, and she wants to figure him out, so badly that she stares even after it isn't possible.
So, the angel could be imaginary. That's a legitimate reading. But it isn't the most likely reading. The most likely reading is this is saying how badly Elisenda needs to understand.
https://www.ndsu.edu/pubweb/~cinichol/CreativeWriting/323/MarquezManwithWings.htm