We're talking about different things. The rules stated the thread width should not exceed 270mm throughout the sessions. It's not about degradation. What happened was that they had a tyre that whilst in use at a high speeds expanded so they had more surface touching the asphalt. That resulted in more grip. The FIA basically had a flawed measuring point in only measuring it before the race as this was no way of making sure they would not expand beyond 270mm throughout the race.
The tyres were found to have wear marks outside of the allowable tread dimensions but the defense for this was that the side of the tyres were being worn by contact with the kerbs, that's a degradation issue. I haven't heard anything about the tyres getting wider at speed. In fac,t as centrifugal force increases the diameter of the tyre at speed it would surely, if anything, peel the edges of the tyre up off the road and make the contact patch narrower.

