I know I already have a disagreement with Caledonian in this matter, because earlier this month he wrote here that existence is relative and depends on the possibility of interaction, something I would never say, because it confuses existence per se with something like knowability

No. That is completely wrong.You are confusing what things ARE, with what we THINK them to be. This is a common problem when dealing with models of reality (in which we exist), models in which we (as external observers) 'know' everything about the model; we tend to confuse the 'we' within the model, which is ignorant, with the 'we' looking at the model, which is essentially omniscient. (This is a simplification, as the full details are beyond the scope of this thread.)

Whether a thing exists has nothing to do with what we know. Whether we can assert that a thing exists has everything to do with what we know. It is entirely possible for properties of the system to be forever unknowable for particular entities within that system, or even entities in general within that system, and still exist. But those entities are not entitled to make any claims about the existence of said properties.