A weak theory. Nothing ever appeared in nature until... it appeared. Coronavirus is a mutation, of the same ilk as the mutations of all the influenza strains are. Which is kinda by definition not seen in nature before. Thats why we struggle every year to produce a useful flu jab to protect the vulnerable. Its why this strain is so nasty, we never had the opportunity to develop resistance to it. Once we do, oft called 'herd immunity', and develop vaccines then it will subside into a nuisance same as other flu strains. Until the next mutation occurs.
Its not alone in mutating : lots of viruses do. Thing is 99 mutations might not mean a thing to us. The hundredth kills us.
If it was deliberately manufactured : not only unproved but dismissed by some of the smarter minds in that field of research, then why release it? Seems a bit of a daft way to cull a population. Far too random and uncontrollable and why wreck your internal economy in the doing of it?
So, even if the Chinese Politburo fancied that as a solution to over-poulation or an unbalance population there are far more effective and contrable means at hand.
An accidental release? Unlikely but not impossible. A Brit lab managed to release live Foot and Mouth into its own drains a few years back. Which is why research labs have to follow recognised international rules these days. Rules and practices which are audited and monitored. So, possible but first the labs would have to be researching that strain at the time or prior to it showing up and the audits would show that. They dont.
But we should never discount the final theory. Aliens!