I know about the first definition as I used to be in Chemistry.Well, sysnthesis is the artificial execution of chemical reactions to obtain one or several products.
It is literally making something up out of different parts.
Or in writing, synthesis refers to combining multiple sources and ideas. Using information from several sources to create new ideas based on your analysis of what you have read.
The evidence has been made from reading, and combining different evidence from other studies.
Again, it's literally been made up from different parts.
And the evidence they did read was of low to moderate certainty in the original studies. Certainly no where near certain.
However, my point was, once again it's yet another meta-analysis, which actually states it's own validity is built on questionable data, and published in the BMJ, rather than any meaningful research.
I don't agree with your other conclusions. I think you're extrapolating too far.