It's worse than "very ignorant". It stinks malice and stupidity at the same time - because the person is rushing conclusions (aka assuming, aka making shit up) about another person, based on little to no information.
I never saw this in real life, but if some acquaintance told me that they avoid dating people without social media presence "because it's a red flag", I'd look for further signs that the person is unjust and/or assumptive and consider avoiding them altogether.
Classical Physics breaks in three situations: if it's too fast, too massive, or too tiny. To address that, new theories appeared. Among them:
What researchers are looking for is a theory that is able to handle all three things at the same time, superseding both the relativities and the quantum theories. That's the Theory of Everything that everyone is looking for. (Except me. I'm looking for my cup of coffee.)
And most people look for it in a specific way: they try to adapt relativity to quantum phenomena. Those researchers however are doing something different: they're imposing a limit on the quantum theories, saying that they break under specific situations, because spacetime would work more like in classical physics than like in quantum mechanics - in other words that quantum theories need to be fixed to relativity, not the opposite.
The researchers then devised a stupidly simple experiment to test their hypothesis out.