It's not "mistake" singular, it's actually mistakes.

The first mistake is the $500 million rover is fried.

The second mistake is believing the first mistake.

(or put another way, the first mistake cost $500 million, the second mistake which they didn't realise at the time, saved $500 million)

But you can't explain the second mistake without first explaining the first mistake, hence the title.

The title is simply clickbait, you're just doing mental gymnastics. It would only make sense if the outcomes of the actions were much more spaced out.

I guess the idea is that it potentially could have cost $500 million therefore it was a $500 million mistake. It's not exactly accurate but it does help contextualize the gravity of their mistake.

Reminds me of my favorite George Carlin bit[1]:

Here's one they just made up: "near miss". When two planes almost collide, they call it a near miss. It's a near hit. A collision is a near miss.

[1]: https://www.quotes.net/mquote/35854

It was feared to be a $500M mistake.