Maddison scored twice inside the opening 20 minutes to set up the result.
He later said: "As you get older you don't like to get reminded that is your birthday because it is a reminder that you are getting on a bit. But his is definitely one that I'll remember.
"It is very rare that you get results and days like this, especially at the champions, here at City, against a Pep Guardiola side, so you have to enjoy it.
"A point to prove to who? I wouldn't say that was the correct wording. You have a point to prove in every game that you play, that is the mindset that you've got to have. I came out of the team for a couple of games but that gets spoken about more in the media than between players.
"If the manager wants to leave me out and play (Pape) Sarr and (Dejan) Kulusevski for a couple of games then that's fine by me. I'll just make sure that whenever I get the chance he can't leave me out. No player is happy not to play.
On his performance, he added: "I felt good out there, I have felt good for the majority of the season bar a couple of bad games recently, but that can happen.
"Really happy with my performance and that of the team as well. We had to defend, we had to rely on Vicario too. When you come here you are under the cosh a lot because of the quality they have got but we stood up to it and Vicario made a couple of big, big saves. Good day all round."