There are at least a couple of suspicious points in this study: First and foremost the central claim is that 5 potential restriction binding sites versus 2 means that SARS-CoV2 is non natural. That does not necessarily follow. Just as SARS-CoV2 is unusually infectious and damaging to humans it could just happen to have an additional 3 restriction binding sites. So there is nothing inconsistent with natural selection…
"Additionally, because the final product does not have a Type IIS restriction enzyme recognition site, the correctly-ligated product cannot be cut again by the restriction enzyme, meaning the reaction is essentially irreversible"
https://en.wikipedia.org/wiki/Golden_Gate_Cloning ----
The choice of focusing on a particular RE pair also smells of p-hacking. Their claim that BsaI/BsmBI makes for easy mixing/matching genomes doesn't make sense in this day and age, when you can use other techniques to make hybrids more effectively (eg, you are not restricted to the natural location of those restriction enzyme sites)