As above, think this falls under serious foul play (definition below):
SERIOUS FOUL PLAY
A tackle or challenge that endangers the safety of an opponent or uses excessive force or brutality must be sanctioned as serious foul play.
Any player who lunges at an opponent in challenging for the ball from the front, from the side or from behind using one or both legs, with excessive force or endangers the safety of an opponent is guilty of serious foul play.
In short, it's difficult to separate the act (the tackle which endangers the safety of an opponent) from the outcome (Elliot's ankle being displaced which has clearly endangered his safety).
Note, there's absolutely nothing on intent in the definition, therefore an accident is equally as punishable as an intentional foul.
I do wonder if the ref has second thoughts initially about whether the tackle endangered his safety but clearly picks up the outcome (from Elliot's obvious serious injury) and therefore shows the red?