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.