I’ve had a search but can’t work out why it’s special.
"why didn't they have a hot-spare" They do! Flight spares are complete, flight-rated copies of spacecraft built for exactly this contingency: https://en.wikipedia.org/wiki/Flight_spare After launch the flight spares are used for terrain testing and troubleshooting. (The "mars yard" has flight spares for Curiosity and Perseverance https://www-robotics.jpl.nasa.gov/how-we-do-it/facilities/ma... which were used to test some wheels to destruction after Curiosity started showing some wear https://www.planetary.org/articles/08190630-curiosity-wheel-... )
The blog post lays it on a bit thick with the $500 million number and the "launch only two weeks away" given that the article itself is illustrated with a photo of the Sojourner flight spare. Spirit had the SSTB1 test rover. If he had actually blown out the entire electrical system, they could have launched it instead. Swapping out the entire vehicle right before launch would have been an awful job, but it's not flat out impossible.
I liked that other people pointed out that risk could have been eliminated by using polarized connectors (I hope they started doing this after the incident), but also made me wonder about "back-EMF" caused by solar flares. In other words, maybe all thick wires and ground/power planes should be hardened against current surges simply due to a solar event hitting mars (which may incidentally cover the case of back-powering the driver circuits).
(One of my standard end-of-interview questions is "how easy is it for me to trash the production database?" Having done this previously[1] and had a few near misses, it's not something I want to do again.)
[1] In my defence, I was young and didn't know that /tmp on Solaris was special. Not until someone rebooted the box, anyway.