I was thinking it could be to take down the Celestials (would make Roger fighting him weird since Roger freed some slaves, but it could be Rocks would've killed the slaves too to take out the Celestials, so Roger opposed him. Garp agreed to let Roger free the slaves (ie take the legal property of the CDs) for his help in the marine mission), but if it was just for the treasure then it's just for their own gain. Unless that's just the official government/marine story and the real reason was something else.
Did Linlin or Kaidou ever talk about the incident? They could know Rocks' real motivation. Maybe they would say something if they heard the official story and knew it wasn't true, but probably nobody's brought it up to them. Those fruits (paw-paw and dragon) don't seem like much and it's weird Sengoku wouldn't probably specify "two" treasures and say that they were taken, if one is Kaido's fruit and Kaido/Linlin's alliance was the reason for telling the story?
There's this chap 1096, where Garp says of course they'd come to take the thing back. Which sounds like the Celestials stole something from Rocks' island and put it on God Valley. Could be a poneglyph. Could be the hat. Considered to be implies it's known to many pirates though. Maybe the CDs kept it, maybe Roger kept it or maybe it's back on Pirate Island now, with Blackbeard.