I was told verbally of a job posting before it was ever typed up. The applications weren’t given to be filled out until it was actually posted. When filling it out I ran across the question “how did you hear of this position “.. I froze. I lied and said internal job posting because I didn’t know if putting “my supervisor told me “ was a good answer or not. Now regardless of what would’ve sounded better I regret lying and feel undeserving of my position. I’ve held d this position for over a year and feel convicted for that minor lie. Some say it wasn’t a lie and some say it was but no big deal. I know that answer wasn’t why I was given the position but still.
I'd say you're overthinking this - it's unlikely to ever come up again or to have had any real bearing on the recruitment process. Most likely the question was simply there for tracking how effective their means of communicating the existence of such positions are.
Carry on and enjoy your new position.
The purpose of asking how you found out about the job posting is often to work out what their most lucrative way of job advertisement is, and use this to help when advertising future job posts.
Whatever you put in that box would not have influenced their decision to give you the job. Some won't have even read what was in that box. You didn't have any advantages over anyone else, you waited until the job was open, and applied. They hired you based on your application and proven skills, not how you found out about the job.
as your name suggests I feel that you are over thinking this a bit. if you didn't fill out the application till after the job was posted publicly then you had no advantage over external candidates and like you said you beat another interal candidate because you had a better interview and they would have had the same (if any) advantages as you. your supervisor wouldn't have told you about the job if they didn't think you were good enough and you also wouldn't have got the job. so relax