There a bunch of speculations here, and not too much knowledge about German Post.
First, Post Offices could possibly fulfill Socialising Criteria, but instead of arguing with current criteria, usually it has grandfathered in from old Ingress OPR guides (before Wayfarer re-branding), that Post Offices generally fulfill it because it connects people to the world.
See here: OPR Candidate Action Guide - A Wayfarer History Lesson
Now, these old guides are often not valid anymore.
If I were to look at today’s criteria I would be more doubtful about German Post Offices in particular, but that’s not too relevant here.
Deutsche Post Group owns DHL. Seeing those two logos together at post offices is a very common sight and does not mean that it is not a normal post office. As such you can not argue for ineligibility solely by seeing the DHL Logo here, it will depend on the store itself.
Usually when people reject post offices here, it’s either lack of knowledge of criteria or because they disagree personally and reject it as Generic Business. They are not commonly accepted here.
Now with this one being rejected by ML, I highly doubt it is capable of mistaking this as a post box instead of an office, it’s more simply that nothing eligible was recognized.
Why is there a post office in the electronics store? Many Deutsche Post locations are not run by them actually, but instead, are run by local partners. This can look like having a second counter in the unrelated store for postal services and items you can buy, or only having the post scanner at the regular store counter and stuff.
Except for the unnecessary part in the description saying it’s publically and safely accessible - that should only be in the supporting information - you made a good nomination attempt here and appealing it was the correct choice. Often there’s room for improvement, but sometimes the ML just rejects anyways. Let’s see how it goes, I’m doubtful that this “Post Office inside other store” would be accepted.