I deal with traffic safety routinely in my line of work. Without seeing the tail lights of the FedEx truck I wouldn’t even speculate about what was happening there. If black ice was the cause then he could have been standing on his brake pedal to no avail.
If you realize that the highway is icing over it’s as much a clue to stay off the roads as high floods or standing water on a highway is.
Here’s some tail light footage from early in the pileup.
https://www.youtube.com/watch?v=dt7rxczzMSE