> maintaining a fork is roughly the same as maintaining a feature flag.

He didn't say that in the interview. Or, he didn't make nearly as broad a claim as you have made. He said:

>> If you want me to maintain a flag to remove it, I can ask you to maintain a fork removing it. Telling people to “fork it” often upsets them.

The context of his statement was people wanting a feature (search as one example) removed (or removable, via feature flag). In that case, the fork is about as hard to maintain as the feature flag, assuming the software is reasonably well organized.

But in general, your claim is not true, and it's not what he wrote.

> You should address the point Mitchell made

No, I have no obligation to respond to something that he didn't say.