Where were Tessio's men when he was bundled into the car for his last ride
That's a detail FFC no doubt didn't want to deal with. But...
Since Tessio's plan was regicide--killing the Don--secrecy would be the top priority. It'd be in his interest not to tell anyone else about it except the would-be assassins. And, to assure secrecy, they probably wouldn't have been from among Tessio's ranks. He would have recruited outside assassins. This is routine Mafia practice when Dons are whacked