I think most people are talking more about arm strength and the feel for different throws than accuracy - although ideally it's both. Ganz was great, but he did not have a great arm. Same story for Zac Taylor. Can't say that I've noticed the "20 years" thing, but probably just an arbitrary addendum to make it seem more believable. But in this case, I don't think there's really a doubt that Raiola has the best arm from a talent perspective we've ever had.
As far as who Raiola is beating out for that distinction - Tanner Lee is understandably someone we all try to forget, but he had an arm. His actual performance was mediocre to bad, but he still got drafted in the 6th round solely because he had a good arm. Armstrong's arm was, well, strong - but not a lot of feel for throws. I'd group 2AM along with him. There aren't a lot of good throwers further back for a lot of reasons - we had a run-heavy offense starting in the 80s, and when we threw the ball more than average in the 70s it was just a different game. QB development and training just wasn't a thing at the college level.