Just straight bullsh*t. They may look alike, but at the end of the day, Jayden knows they're different people, and he treats them as such. If he thought of him merely as a substitute, he wouldn't be dreaming of Diego. Plus, in the beginning chapter, when they had s*x, Diego saw Jayden's dog tags, he asked about it, and knowing Diego was there, he literally threw the dog tags aside, the thing that is the literal memory of the past with his past crush, aside, because he wants to move on and live in the moment with Diego. In the moment of confusion of meeting the past stain (Daniel) it put him in a sense of disarray because he's currently fighting to live for himself and enjoy his life only to remember the things left behind, and with his insecure mind not yet stabalized, he's easily influenced by an outside source because he feels he can find comfort more in the words of something more known, than being with the unknown as he's afraid to be happy again.