The argument is identical to arguments supporting intelligent design, that because it evolved in an unlikely or complex way, it must have been intentionally created. There's a reason we haven't seen a virus like this in a century - because it's so unlikely that a new virus will appear in any given year with both a high rate of transmission and a high fatality rate.
This is bad logic. Suppose some state sponsored lab actually did create a hybrid virus (not terribly hard, as demonstrated by the controversial ccr5-binder/flu fusion which was generated in Australia iirc) what would be the evidentiary standard you would use to judge that this 1) could be or 2) was the case, of course, assuming you could not have access to the state's lab logs?
Or would you throw up your hands and say "it's unprovable"?