Sollozzo was going to meet face to face with the son of the man he'd been trying to kill. So even though he misread Mike as a civilian he still would have been initially on high alert for ANY sign of danger. Once he's picked up Mike, frisked him and possibly thrown off any tails, he's fractionally more relaxed. And that's all the edge Michael needs.
As it turned out Sollozzo was in the car but 1) Sonny was going more by intuition than planning. Over time planning is going to be more effective and 2) The Family had ONE chance to get Sollozzo.
It was better to get very close and do it right than have Michael (or anyone else) try to shoot someone in the back seat and possibly be shot himself, run over by the driver or worst of all fail to do the job on both McCluskey and Sollozzo.
"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming
Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.
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