So, if Sollozzo would have tried to kill Vito regardless of Sonny's malaprop, then he must have figured he could do without Vito's political and legal connections. Given everything they knew about Sonny, with Vito murdered, would Sonny have just said "oh well,I'll miss Pop, but business is business"?
oh NO!!! no way Sonny would do that. But I believe DC and DTs's points are that Sollozo was really ignorant to the state of the Corleone family after the assassination of Vito.