A methodology for a systematic review should be presented in full detail, according the the PRISMA reporting guidelines.
The authors have here given minimal indications of their method. How many reviewers reviewed the studies? How many were found on the initial searches from each database? On what grounds were the non-included studies disregarded?
Without this information, readers cannot repeat the method, or even judge how adequate this review is.
I would recommend a complete revision of this publication.