This was a case of the Don not telling his consigliere everything. Tom had no clue that Luca had been sent as a ouble agent, and he tells Sol that he may calm Sonny, Tessio and Clemenza down, but that there would be nothing he could do to stop Luca Brasi. In a great bit of acting, Sol smirks and says something to the effect, well let me worry about Luca. Of course he already knew Luca was swimming withthe fishes.

BTW Tom's tipping his hand by mentiioning Luca's fierce loyalty gave Sol and Barz another insight into the Corleone operation's lack of communications.