It doesn’t seem to have opened the floodgates to malware on the Mac. Perhaps this is more about controlling the platform and ensuring all of the money goes through them.
His quip about it taking “away user’s choice of a more secure platform” is just nonsense. If users want to exclusively install apps from the App Store, they can. Given how difficult it is to install unsigned apps on the Mac, I doubt many will be “tricked” into sideloading malicious apps. And there’s still plenty that Apple could do to mitigate the issue while still allowing for the installation of apps from outside the App Store.