Nowadays when you see articles results of new research of covid19 in the media, those articles often include 'hasn't been peer reviewed yet' or 'reviewed by other scientists' or any such verbiage, either as a disclaimer or as 'now it must be true'. But that's not how it works; it's not because something has been 'peer reviewed' that it's 'The Truth' or 'Real Science'. Peer review, in reality, just weeds out (most) quacks (although in the OP's case it seems it didn't even do that) and checks that the paper is not completely out of touch with what is happening in and known about the field. It's not QA of the work itself.
(I don't care to debate if it should be, and if more money should be spend on replication etc, just providing some real world context on something that is quite opaque to and often misunderstood by those not in academia)
That's the theory. The reality is that there is no in-depth review. You're lucky if a reviewer actually reads the paper all the way through, let alone checks the numbers and applies a level of critical thought to the methodology, analysis and conclusions.
The authors' behaviour is outrageous, but this story is also about a broken reviewing process, partly due to wrong incentives.