That was an Oda's silhouette, It has almost zero real reliability.
Could have been a 2vs1 as a 1vs1vs1.
In fact, in Sengoku's mental image/silhollouette It was a 2vs4:
That was Xebec's secret personal goal, but his own crew had a different One and he can't just force them to do otherwise when he lured them at GV exaclty because of the treasures and Shakky.
Now Roger is going were the treasures are :
Here they'll met all the Rocks pirates, who are going to see that Shakky Is with them too.
Then Garp Is going to arrive.
Then all Is starting.
I agree they won't beat Xebec since they can't be the reason he won't save his wife (at least), but that Is because Imu/Gunko Is coming to kill him in the end.
That's why he has Xebec's sword in the flower room.