The suggestion rule does not overlap 100% with how Gaijin handles its own additions, you can find statements by Smin to that effect too. The rules simply exist to regulate user suggestions.
Like you yourself point out later, stuff like the O-I or the GSR are technically suggestible, but cannot be implemented. The other way around also happens, with Gaijin “breaking their own rules” and adding ahistorical things, one example of which can be found below in this post.
But in any case, let’s go with the actual suggestion rule: at least one major component unique to the vehicle needs to have existed.
For the Flakpanzer the result is that you have to lawyer about whether the guns, which were built in metal, qualify as unique. You can say no, because they were also meant for ships. You can say yes, because no other ground vehicle was ever intended to mount them (and in point of fact such guns are indeed absent from the German ground TT).
There is no escaping the fact that the final decision for or against is always going to be arbitrary, because if Gaijin wanted to add a vehicle based only on an existing gun, they would, and most of the community would not bat an eye.
Time for an example of Gaijin breaking their own rules.
The Tiger II Sla should then be removed from the game, you realise. The Sla engine was developed for the Jagdtiger, and was only ever installed on the Jagdtiger. The idea to fit it into a Tiger II was pitched late in development but installation could not be carried out without a rework of the engine deck, which never happened.
Just like the Gerät 341 were never mounted on a Panther and a turret to mate them was never built, the Sla engine was never mounted on a Tiger II and the engine deck to mate them was never built.
Hilariously, just like the Flakpanzer with the Gerät 341, the Tiger II Sla cannot claim that a major component unique to it was built, because even in the most generous interpretation, it would share the engine with the Jagdtiger! Just like the Gerät would be shared with the Kriegsmarine!
There is no component unique to the Sla, built for the Sla. It is a Tiger II in every respect bar the engine, which was not primarily meant for it, developed for it, or ever installed.
Yet one is among the most popular mid tier premiums in the game and the other is unobtainium.