Sebi was upholding the site rules in the sense that there's a long-established precedent that if a sender uses tracking and the tracking shows that the package arrived, then the sender has fulfilled his or her side of the trade even if the recipient claims to have not received it. The reason for this precedent is obvious - if there's some sort of error at this point, it's *much* more likely to be the recipient's fault than the sender's, and so as a rule we assign the loss to the recipient. This might seem unfair in cases where the recipient is innocent, but *someone* is going to take a loss here and it might as well be the person who's clearly most-*likely* to be at fault. In this case, that would be you. No one at the site wants to interrogate your postmaster over this - that's up to you so that you're properly incentivized to sort these delivery issues out.
This is how it's always worked. The chef analogy is silly because the relationship of traders on this site and shipping companies is not similar to that of a chef hiring a waiter - we can fault a restaurant when a waiter drops your food, we cannot fault a trader on here when they use tracking and a package disappears, which of course opens up the possibility of people abusing the system by claiming to have not received tracked packages.
The one point in your favor on this is that it's not clear what constitutes "proof of delivery" in the rules here. Most people, including me, consider tracking to provide proof of delivery. You're right, though, that there are stronger ways to provide proof of delivery through shipping confirmation. I'll note, however, that one can claim that those don't provide "proof" as well if one pleases - one could just say that any confirmation is erroneous (as you did with the tracking case) and that therefore the sender is still liable for delivery. It would probably be good to make this explicit in the rules.
The bottom line here though is that this outcome is not sebi's fault. He's applying the precedent that everyone understands and will reply to this thread telling you exists. There are very good reasons why we don't make traders resend packages even if tracking shows that they arrived, and thus changing the rules on this would be a bad idea. People in BTRs need to understand that even if they're innocent, then in many cases *someone* is going to take a loss and if the circumstances are such that it's much more-likely that you've done something wrong than your partner than it makes perfect sense for the rules to be structured so that loss is placed on you. It sucks in that particular case but it would suck a lot more if the rules were different.