The author is contradicting himself: said that virus engineering is done by splicing of existing other coronavirii, and then also said that the two codons CGG is very unusual for coronavirii. Splicing is also naturally done everywhere and all the time, just by natural genetic events, so if there was splicing to create the two CGG codons, it can be natural.

So still nothing to point out to bioengineering in my opinion.