I think that's generally correct, but I'm not familiar with all of the rules. That said, even if the Southwest plane had been faster on the runway, they weren't going to be able to maintain 6000 feet of separation. If Southwest starts rolling from a dead stop, and Fedex is barreling in at 140 knots from two miles, they're necessarily going to converge until Southwest accelerates, which takes a minute.

Juan Brown [1] made an interesting point also. For a Cat II or III approach, and this one was definitely Cat III, there is an ILS critical area that Southwest would have impinged on as it was taking off.

The controller very clearly made a huge mistake.

[1]https://www.youtube.com/watch?v=SvUOHa8n7aQ