I think Tom was referring to the fact that Luca was totally and exclusively loyal to Vito, and reported directly to him--not through Tom, Sonny or the caporegimes. We saw this when, after the Sollozzo meeting, Vito said to Tom, "Send Luca Brasi to see me." And when he gave his order to Luca to "go to the Tattaglias, make them think that you're not too happy with our family, find out what he's got under his fingernails," he told it to Luca alone, without Tom or anyone else present. Therefore, Tom probably was telling Sollozzo the unvarnished truth when he said, "Even Sonny won't be able to call off Luca"--Sonny had no authority over Luca, and Luca would have been so outraged over the killing of his Don that nothing would stop him.