Half true. He may have believed that after some time. But he didn't believe it at the time of writing the Science of Hitting. And he didn't believe it as a player. If a pitcher hit him inside with a fastball he wouldn't try and hit it the other way because of the shift. Because it's counterproductive. That being said, if someone threw something outside to him he wouldn't try to pull it either. Williams was a fantastic hitter.
From The Science of Hitting:
"I liked the bat so much I started the season with it, and right away I was getting
hits into the spaces they opened for me in left field when they used that tough shift.
I wasn’t getting around quite as fast with the heavier bat, but against the shift it
was perfect."
...and...
"When I had such a hard time with Boudreau’s shift, and ones like it that sprung
up in 1947 and afterward, I survived by learning to hit to left field. Everybody was
saying—and the Boston writers were writing—that I wasn’t trying to hit to left, that
I was too stubborn, that all I cared about was ramming the ball into the teeth of
that shift, getting base hits in spite of it. The fact was, I was having a hard time
learning to hit to left. It wasn’t because I didn’t get any advice. Of that I got a
truckload.
Ty Cobb wrote me a two-page letter, outlining how he would do it. We met at
Yankee Stadium during the 1947 World Series, and he took me around behind a
telephone booth and we talked. He said, “Oh, boy, Ted, if they had ever pulled that
stuff on me, that drastic shift . . . ,” and his mouth was watering, seeing in his
mind’s eye the immortal Ty Cobb lashing the ball into that open range in left field.
Well, Cobb was more of a push hitter, a slap hitter. He choked up two inches from
the bottom and held the bat with his hands four inches apart. He stood close to the
plate, his hands forward. He had great ability to push the ball, to lash hits all
around. He was a great athlete, maybe the greatest, but he was a completely
different animal from me, and his words were like Greek.
The arc of my swing was much greater than Cobb’s What he said would apply to
guys more his type, guys who choked up on the bat more and pushed the ball
around. That wasn’t in me. I was down, with a longer stroke, a greater arc.
When I beat the shift, I did it by taking my stance a little farther from the plate,
striding slightly more into the pitch—but concentrating on getting on top of the ball
and pushing it. A push swing, an inside-out swing, fully extended, the hands ahead
of the fat part of the bat. This produced contact at 90 degrees or more from the
direction of the pitch, and sent the ball to the left of the pitcher’s box, away from
the shift. Almost like hitting pepper."