It was more likely putting evolutionary pressure on Coronaviruses in order to change it's properties and behaviour.
Regarding selection pressure people do that all the time, in fact it is far from easy to artificially apply environmental pressure and obtain highly functional new variants. In cancer research, xenograft are extremely tricky to do with immunodeficient animal models, let alone human tissue in the lab. Sure you can have some success but it's difficult enough to reproduce. His point is that even if an artificially selected highly viable variant was obtained, it is very unlikely it can prevail in real world nature selection (large scale too). I'm no expert but happy to read about any paper if it's not the case.