Rejection of a paper in peer review is normal (even common! most real journals reject well over half of submitted papers... social justice journals, not so strict, but that is another issue)

And rejection comes with explanation(s), usually 2 or 3, in detail to some extent or antoerh, from the peer reviewers and the journal editor.

Rejection of a paper after passing peer review is unusual, especially with no explanation.

A steel man argument would be some problems with the peer reviewers and the editor doesn't want to publicly explain the embarrassing details, or something like that.

The opposite argument is the paper drew attention in pre-publish, and "respected" Teh Sceince!tists came down on the editor like a ton of bricks to reject the paper and give it a public stink, so while the authors are free to look for another journal, those journals under sway of Teh Science!^TM will look a the paper and the authors with stink eye from now on....

Given all the lies and shenanigans from Teh Science!^TM over the past 2 years, hard to believe the steel man even happens anymore concerning anything that goes against Teh Science!^TM regarding COVID.

Also, there is the little problem of many journal editors being Chinese nationals, who are totally not under the sway of the CCP ;) .. but again, that is another issue.