金剛經纂要刊定記 4.332

4.332

無賊者，意以煩惱為賊。謂斷人慧命劫功德財，致使行人失於聖道，流迸生死曠野，不達涅槃寶所，為害頗深故名為賊。

As for "without thieves": it takes afflictions as thieves. That is, they sever a person's life of wisdom and rob their wealth of merit, causing the practitioner to lose the noble path and drift into the wilderness of birth and death, unable to reach the treasure-abode of nirvāṇa; because the harm is so profound, they are called thieves.