I think the last few posts reinforce what I said above. Tom was telling Sollozzo that Luca was only under the Don's control and that was one variable that Sollozzo need be aware of. Of course, that's why Sollozzo whacked him. As someone said above, the book makes it clear that Luca would never go against the Don and the story of Luca's unswerving loyalty to Vito was well known throughout gangland.