Bullshit reason in my opinion. Of course Stroll would attempt to do anything to get back to the pits, that's his job. And like he said, from inside the car it's hard to see how broken it exactly is. But for anyone outside of the car is was obvious that car was destroyed and wasn't going to come back on it's own in any safe or timely manner. It should have been red flagged right away. Even if Stroll got it going and carried on to the pits, there would still be need for debris clearing and barrier repair. And with the state of that car, he would have put oil and debris all across the track in doing so, requiring even more clean up.
It should have been red right away. There have been so many issues this year where race control is slow to put out a flag or VSC. And weird choices when to use the SC and when to use the VSC. They need to do better in the future.