Tbh, I'm not sure that he's not just saying this because he figured out what Otonashi's doing and decided to help out. Make Yui's biggest dream become real, at least in her mind. It was obvious that she liked Hinata, and marriage was... really kind of not something Otonashi had planned when he set out to fulfill his friends' dreams. So it may have meant even more to Yui when Hinata "stepped up to the plate," if you'll pardon the pun.
Idk, maybe he did mean it:
Whether he meant it or not, he gave her what she needed. Half of the reason Yui was so unhappy was because her mom had to do everything for Yui, and she felt bad about it. Having someone who not only loved her, but loved her enough to do everything for her, was everything she could ever want or need.
And he's right about something: he saw the real Yui. She was probably more open and honest and sincere about everything than anybody ever was. Why wouldn't she be? She'd never been able to be anything before, why not be and do and say everything she can now?
