Okay but if they weren't credited on the game, and there is no evidence that any of them worked on the game, how exactly is it "common knowledge" that it is so? What's the source? Who said it? When? Where? A large problem with information on the internet is that people like to repeat things they heard and think it could probably be true, without bothering to actually learn about the source of the information, and it gets repeated over and over, even though it is not factual.
Claiming something is true doesn't make it true unless there is some evidence behind it. The truth is, the only connection between Radiant Historia and Radiata Stories is in fact the character designer. A character designer who doesn't work for tri-Ace either. But there is also one ex-tri-Ace employee who conceptualized Radiant Historia - Satoshi Takayashiki. He used to work at tri-Ace way back as a planner on Star Ocean Blue Sphere (the gameboy title), and hasn't been with the company for ages. He did not work on Radiata Stories, he wasn't even part of tri-Ace when the game was made. He works at Headlock, and they did not work on Radiata Stories either.
This seems to be the case of the two titles sharing a similar sounding title and a character designer, and also sharing a creator who used to work at the developer who made the other game a long time ago, creating a myth that there was a closer connection than there really is.