The one highly-enthusiastic reviewer is a bit suspicious, but I'd more suspect it was a case of the author recommending a friend as a reviewer (a common practice). But it could also just be by luck they got a person who was negative towards covid vaccinations so wanted to be very supportive.
https://www.mdpi.com/journal/vaccines/instructions
"Reviewer Suggestions
During the submission process, please suggest three potential reviewers with the appropriate expertise to review the manuscript. The editors will not necessarily approach these referees. Please provide detailed contact information (address, homepage, phone, e-mail address). The proposed referees should neither be current collaborators of the co-authors nor have published with any of the co-authors of the manuscript within the last five years. Proposed reviewers should be from different institutions to the authors. You may identify appropriate Editorial Board members of the journal as potential reviewers. You may suggest reviewers from among the authors that you frequently cite in your paper."
Obviously people are supposed to do so ethically, but cabals of friends can potentially hack the process by suggesting each other and working together.
Referees are normally anonymous. But once they start advising to insert some references to some obscure and/or irrelevant articles...
Science should be a process where eventually truth wins out. For this to happen scientists need to be able to publish their findings without fear of retribution. They already risk losing funding, they already have poor job security, they already need to meet a quota for papers and citations. Adding more negative incentives on top of this won't do society any good.