This is a fake, sure. Maybe it's not perfect, but the software is improving. So within a few years, I'm guessing that fakes will be indistinguishable from true recordings.

But this was based on the text of an actual speech written for President Nixen, to be delivered if the mission failed. So isn't it likely that he practiced it, just in case? And if he did, it might have been recorded, more or less accidentally. As Reagan's quip about nuking Russia was.

And so as others note, it becomes crucial to look at the historical context.