I think it's more correct to say that lab leak is a plausible theory. But it's going too far to say it's the most likely theory. I do wish China hadn't clammed up and would have allowed a real investigation. We'll never really know, I suspect.
in which the final claim is "Furin cleavage sites occurred independently for multiple times in the evolution of the coronavirus family, supporting the natural occurring hypothesis of SARS-CoV-2."
Er, that's not really strong support for the naturally occurring hypothesis, it's a refutation against 'naturally occurring is impossible'.
The spontaneous evolution of furin sites is rare in the specific clade of SARS-CoV-2, so if it's support it's very weak. The phylogenetic trees presented in the paper are VERY much loaded to obscure this fact (omitting betacoronaviruses in the alpha-delta tree, and in the breakout betacoronavirus tree, arbitrarily loading up multiple copies of "basically the same" strains which all present a furin cleavage site to make the pie slice of furin site-presenting sequences bigger).
On the other hand, copy-pasting interesting motifs into a different but related genetic entity to achieve GOF is basically "the first strategy you do" as a synthetic biologist. Hell, even I did it (to gain function in a non-pathogenic system) https://pubmed.ncbi.nlm.nih.gov/24934472/ "we substituted one of four amino acids (Asp, His, Asn, Gln) at each of the 12 ligating positions because these amino acids are alternative coordinating residues in otherwise conserved-cysteine positions found in a broad survey of NiFe hydrogenase sequences."
Given there's no other plausible explanation ("bat soup from wet market" theory is completely laughable to anybody who bothers to look closely, and there's no viable path identified that doesn't involve Wuhan labs in one way or another) - it's as much "just a theory" as evolution is "just a theory". Sure, if you work really hard and ignore a lot of facts and have very active imagination, you could imagine some alternative scenario. But the level of proof and agreeing with available facts in this theory would be way lower than in the theory that admits it came through the Wuhan lab. How did it happen - which accident led to it, which rules were bent, who didn't report feeling sick and who was bitten by a bat but didn't want to talk about it out of fear of losing their job, did messing with the virus genome and GoF research play a role and how big the role was - we don't know and probably never will. But calling it the most likely theory is not "going too far" - it's just admitting the evidence we have and looking at it objectively and not trying to fit the facts into predetermined conclusion because we don't like the one that the facts suggest.
