I'm not a virologist like Sorensen, so I cannot really make the argument, but I don't think that "His whole supposition is because it's more infectious than most viruses" is a fair representation of the article.
“There are several factors that point towards this,” says Sørensen. “Firstly, this part of the virus is very stable; it mutates very little. That points to this virus as a fully developed, almost perfected virus for infecting humans.
“Secondly, this indicates that the structure of the virus cannot have evolved naturally. When we compare the novel coronavirus with the one that caused SARS, we see that there are altogether six inserts in this virus that stand out compared to other known SARS viruses,” he goes on explaining.
Sørensen says that several of these changes in the virus are unique, and that they do not exist in other known SARS coronaviruses.
“Four of these six changes have the property that they are suited to infect humans. This kind of aggregation of a type of property can be done simply in a laboratory, and helps to substantiate such an origin,” Sørensen points out.
He is not the only person who is on to this, nor would this be the first time a virus leaked out. Took a decade I believe before the truth came out.
We may never know with 100% certainty, but we will know much more with time.