I was going to cover some of this in my post tomorrow but essentially: In coronaviruses, the receptor binding domain (RBD) of the spike protein is the region that contains the most variation between viruses in that family. (That is, when a member of the coronavirus family mutates into a new species, that mutation always involves Single Nucleotide Polymorphism (SNP) in the RBD of the spike protein.)

There are a half dozen residues in the RBD that serve as the important "binding agents" to the ACE2 receptor. As one would expect from our experience with the rest of the coronavirus family, five of those six residues in SARS-CoV-2 have mutated from their SARS-CoV progenitors. The important thing to note is that with these five mutations, the binding affinity of the SARS-CoV-2 RBD is lower than for SARS-CoV.

No one setting out to engineer it for devious purposes—such as for a biological weapon—would have created an RBD that was less likely to result in an infection that the original form that the virus had.

It's also noteworthy that outside the RBD, the spike protein in SARS-CoV-2 is very very similar to the spike protein in SARS-CoV.. an indicator that the handful of changes in the RBD are the product of evolution, not an engineered origin.