It's very common for journals to ask for reviewer suggestions. For example I guess this is the journal in question: https://www.mdpi.com/journal/vaccines/instr

It's very common for journals to ask for reviewer suggestions. For example I guess this is the journal in question: https://www.mdpi.com/journal/vaccines/instructions 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.