Sol's compliments are presented immediately after Tom reveals the connection to the Tattaglias, so if Sol is buttering Tom then that discovery is what he's buttering him with
Sollozzo / Barzini's strategy could also have been to let Tom discover the Tattaglia connection, hoping Tom would then stop digging deeper Tom obliged!
Sollozzo inwardly feeling smug that the Corleones have no idea about Barzini connection
