I think in general those tackles aren't red cards - the defending player tends not to come through the player/approaches from the open side and wraps their tackling leg around the ball. You see those every week in the UK and they're almost always applauded.
The reason I feel this one is a red card however, is Struijk's trailing leg lands on Elliot's rather than on the ground, and dislocating his ankle in the process. If you're tackling off the ground it doesn't really matter if it's your studs or knee that causes the injury.
It's not malicious and those tackles are normally successful, but this time he wasn't able to control it fully and caused serious damage in the process.
A dislocated ankle is a much more favorable outcome than fracturing tibia and fibula - I remember hearing Alan Judge's leg go at Ipswich, and the spiral nature of the fracture kept him out for a year and completely undid his career. Elliot will most likely have a fair bit of ligament damage, but could be back within a few months if it's a clean dislocation.
Unfortunate for the guy, he was looking very good so far this season.