I did, however, once know a fellow that did tech support for an ISP, that used his work calls to source his freelance onsite tech support business. Once the main company found out they let him go.
Omitted from the article is whether the OP charged for his time. But I can't help but think that either Apple is ridiculous, or we're not hearing the whole story.
Employee told a potential customer that he will make a home-visit, for free. There is no way that the Apple Store would support making promises like that. But if he made that promise while he was at work, it could be construed that he wasn't doing a personal favor, but acting on behalf of his job.
So if anything went wrong (say, he fried the computer) then Apple could potentially have some liability.
1. don't want to potentially lose revenue
2. don't want to dilute their brand
3. don't want any liability associated with an employee doing work outside of their officially sanctioned capacity
4. don't want to culturally set an acceptable level of noncompliance with procedure even if the above violations aren't egregious.
Insurance is very concerned about claims resulting from off the clock / third party work related to day time work. So if you are a doctor working in a practice group, and then do freelance work, that is going to absolutely freak an insurance carrier out.
Security - people don't realize that a lot of stuff on-premise / on-network is tracked and logged. So if you "help" someone out off-site / off work people get nervous. T-mobile employee doing a sim swap with customer in store on video - sure. T-mobile employee logging in off hours to do a sim swap - maybe sketchier even if they are "helping a friend".