Clearly a case where there was something 'special' about the car. Probably one of a relative few with some particular features or options, or thee was something really unusual about its history. Otherwise, that car wouldn't be worth anywhere near that amount.
Yeah i don’t know what it was- The 429 ‘Boss’ supposedly is worth a bunch of money- not sure how much- I don’t recall them discussing anything special about the engine or anything-
Here’s a page discussing some foreigner selling one for $480,000
https://www.motorauthority.com/news/1089086_is-a-1969-ford-mustang-boss-429-worth-480k-to-you