Because the PRRA insertion is out of frame relative to the aligned RaTG13/MP789 sequences, and insertions produced by polymerase slippage or release/repriming are expected to preserve the reading frame, those natural insertion mechanisms are excluded - leaving recombination or deliberate insertion.
Reasoning
Insertions generated by intramolecular polymerase slippage or by copy-choice release-and-repriming have been postulated to keep the viral reading frame intact, because they copy contiguous template in-frame. The SARS-CoV-2 PRRA insert, when aligned to RaTG13 and MP789, is out of frame - it splits an existing serine codon. Therefore the two commonly invoked spontaneous single-strand insertion mechanisms do not straightforwardly account for it. The remaining natural route is recombination/template switching with another virus supplying the insert; the alternative is deliberate insertion by site-directed mutagenesis - precisely the sort of edit routinely used to add furin sites to CoV spikes. This keeps a lab insertion a live possibility for the FCS.
Validity verdict — approved (checked)
Reconstruction. Premises: (i) polymerase-slippage and release/repriming insertions are expected to preserve the reading frame (copy contiguous template in-frame, respecting codon boundaries); (ii) the PRRA insert, aligned to RaTG13/MP789, is out of frame — it splits an existing serine codon. Load-bearing step: modus tollens — if mechanism M preserves frame and the observed insert does not preserve frame, M did not produce it — leaving recombination/template-switching or deliberate insertion.
Evaluation. The inference is a clean modus tollens and holds conditional on premise (i). The disjunctive conclusion (“recombination or deliberate insertion”) is not overclaimed. The obvious pressure point is premise (i) itself — whether slippage-type mechanisms really must respect codon boundaries (a 12-nt, i.e. in-frame-length, insert placed off codon phase is exactly what critics say slippage can do) — but that is premise truth, priced at step 8, not a failure of the inferential step. Given the premise as stated, no undercutting defeater survives. Approved; the logical step is elementary and traced → checked.