1. A straight smash going to the body can be responded before hitting the wall or even the floor. If you see enough pro matches you will see this happening. If I find some vid I'll share it here with you. [Here he nearly gets it but you can imagine that many times they can respond to it with a bit of reflexes and ball knowledge](https://youtu.be/dnuZ_gU3Soc?si=a9m4pPLiHoSA2hJr&t=473).
2. The 90% is just feel after many years of playing myself and watching pro matches. Not saying the straight smash would be a bad move either, I think maybe from that position and with this players it would still win a 60% of the time, so it's not a bad choice, but a 90% win possibility is better than a 60% one.
[You can see here how a straight smash can be responded. When you see the players are already running forward to get your smash back from the wall, the win rate of goes down. ](https://youtu.be/dnuZ_gU3Soc?si=kNOXXS2-lVjLJaOB&t=423)
At lower levels this smash's win rate goes a lot higher though as players will not be positioned so well to respond it.