Went over to the Seaver link hoping to agree with you :- ) although I'd remembered Tom Terrific as a drop-and-drive guy ...
Aside from the big rock step ... notice how Seaver leans aggressively back to load his weight ...
In the vid you can sense him "gathering" the ball to his belt buckle; you can see he almost looks like he's "protecting" the ball with his entire body and that reflects the fact that the force is organized around, derived from, his CG.
You also have the long stride forward, the perpendicular torso at release, and notice the knee-dragging lunge forward...
Actually would use Seaver as an example of how to get momentum off the back leg...
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Obviously at 97 mph, Taijuan's leverage is not NONEXISTENT :- ) ... just mediocre, 4-out-of-10 .... and he was, after all, 18 years old ...
Much more important than the lack of lower-body leverage, is the fact that his mechanics are so clean and his arm so live...