Credentialism is a form of the Appeal to Authority argumentative form. This is considered one of the worst forms of argument – but it is used because it is effective.
Regarding Covid-19, this form is used to shut down those who raise other perspectives and ask valid but perhaps difficult questions.
Rather than address the issues or answer the questions, the response is to say “I have a credential and you don’t, therefore you are wrong“. That is a logically invalid argument, but the Appeal to Authority form is effective at persuading many.
Bertrand Russell said a fact is true regardless of who says it. He had been awarded a Nobel prize (see what I did there?)
The above tweet implies that “credentialism” is perhaps done primarily by the left. That, though is not true – nearly everyone uses this argumentative form. However, the left does appear to favor rule by technocratic elite, so there is that problem – and that lines up with credentialism.