Furthermore, as was explained in this scan, Turtle, who also uses Speed Steal, demonstrated that the ability can be lethal if you use it to slow down bloodflow. He did this to four guards and put 3 into a coma and the 4th died becaise it caused his heart to explode. So even if Speed Stealong Superman's bloodflow doesnt kill him, it will certainly **** him up.
Scan of Wally proving he can also slow down bloodflow via Speed Steal:
Speed Steal can also be ised to slow brain function. So its pretty safe to assume that Flash Speed Stealong Superman's body, brain function, and bloodflow would put Supes at a major disadvantage. And again, every punce of kinetic energy Wally takes from his opponents instantly amps his speed and makes him more powerful. So while Superman's body, brain function, and bloodflow is slowed down, Wally is simultaneouly amped.
So NO abhi, Wally's statement is MEANINGLESS. Wally has countless ways to kill superman with ease