I think what he meant with those words was that experts think it's impossible to create such a backdoor and keep it 100% safe from being leaked or exploited by bad actors. Which I think we all here can agree with.
But they would be asked to share this back door with the thousands of law enforcement organizations, as well as other countries. If they had to do the same with their signing keys, those wouldn't be secure either.
Now for a backdoor that you're sharing with a bunch of people...