My reasonings

1. Shanks has stronger Haki,  Rocks had a 5KM Haki clash while Shanks had an sky splot which are similar feats.  However, Shank's Wifi Haki feat in Chapter 1055 showcases a level of Conqueror's Haki that Rocks lacked.   In addition, Shank's Haki has been given better portrayal with multiple statements of people saying how massive his Haki is and even the giants compared it to Joyboy's massive Haki.  And better Conqueror's Haki means stronger attack and stronger defense too

2.  Xebec is slower than Shanks, given that Rocks was always equal to Xebec and he never could blitz him we can say that Xebec isnt among the fastest OP characters. Shanks in comparison is one of fhe fastest characters ever introduced by Oda based on multiple feats

3.  Shanks can dominate Xebec thanks to his FS and his ability to supress FS,  this gives Shanks total control of the fight as he can know everything that Xebec can do all times

  
4. Shanks haa higher scaling, Xebec was around equal to Harald while Shanks beat Loki who scales above Harald in Hybrid Form and I would even argue that even base Loki might beat Rocks so Shanks beating Loki migbt mean a big gap between Shanks and Xebec. 

5.  Shank's full power is not revealed yet while Xebec is, it's almost impossible for Shanks's best feats in the manga wont be overwhelmingly greater than what Xebec have done as the powerscaling constantly rises the closer we get to the endgame.   Like just to put an hypocritical here Shanks could end up fighting an Omer Shamrock with a legendary Blade or Blackbeard when he has awakened multiple fruits or maybe even fight Imu himself and give him a good foght even if he can't beat him.  

Is this a hot take?