The guarantee is that the Bocchicchio Family will provide a hostage to assure Michael's safety. Sollozzo pays to have a Bocchicchio family member be held hostage by the Corleones. If Michael is harmed, the Corleones kill the Bocchicchio hostage. Then the Bocchicchios take vengeance on Sollozzo. As the English-language version of the novel states: the Bocchicchios were so primitive that they would let no punishment, no threat, stop them from taking vengeance, and there was no defense against them. "A Bocchicchio hostage was gilt-edged insurance."
The reason Sollozzo's safety wasn't guaranteed was that he was the aggressor, the "hunted one." No hostage could guarantee his safety. That's why he paid McCluskey to be his "security."