A resposta curta é que ele não bateu apenas uma vez, ele bateu em vários vezes, cada vez no mesmo lugar. Ele só quebrou o impacto final.:
“Stop this!” Cooper shouted, his face mere inches from the scientist’s. Mann’s response was to slam his faceplate into Cooper’s, hard, snapping his head back.
“Someone’s—glass—will—give—way—first!” he grunted between strikes.
“Fifty-fifty you kill yourself,” Cooper howled. “Stop!”
And suddenly Mann did stop. He looked at Cooper with an unreadable expression. His faceplate was already riddled with tiny fractures.
So was Cooper’s.
“Best odds I’ve had in years,” Mann told him, and then he butted his head into Cooper’s glass. Cooper heard it crack, felt the cold first, and then the acrid, nose-scorching scent of ammonia. - Interstellar Novelisation