There is visual evidence. He's arguably the best in the world at it alone with ederson. Stats like pass completion are literally irrelevant stats without any context like what the pass attempted to do, what is the goalkeeper asked to do, who are the player targets and how well do they compete for long balls, etc. Unfortunately with goalkeepers, the publically available stats like post shot xG on fbref or pass completion or whatever else are literally irrelevant stats. Some people do advanced analytics for goalkeepers, but it's not publically available unless they post little snips like John Harrison on twitter. Otherwise, you're wasting your time looking at fbref or Fotmob stats about a goalkeeper. For the general public, you just gotta judge it visually and each incident individually.
The CL final performance is the highest possible game to show your stuff, and Onana showed a passing range and level that very few are capable of. Raya is not capable of that level of passing.