Why does the Thrasher tell K. he shouldn’t believe everything the guards say in “The Thrasher”?

The Thrasher explains that the guards’ fear of being beaten has made them “a little feeble-minded. ‘What this one here, for example,’ — he pointed at Willem — ‘told you about his possible career is quite ridiculous. Look how fat he is, the first strokes will be wasted on fat.’”