The same way Lamar Jackson's accuracy got better and Josh Allen's: repetition and refinement of mechanics. LJ completion % at Louisville was 54.7%, 56.2% and 59.1% respectively. His TD/INT ration and Y/A weren't that great for an NFL prospect. Josh Allen was 56.0 and 56.3% in two years as the starter in Wyoming. Yet, both are MVP-type players at their position and win games throwing the ball.
Now, if you have a measurable monster like Colin Kaepernick who needed major refinement in throwing mechanics and had a long release, there's more reason to worry. But the thing to find hopeful about Richardson is he's pretty short, compact and fluid—not much wasted motion at all. So projecting out, it's easy to see him getting more under control with experience and repetition, because he doesn't need to revamp his mechanics. And he got a canon, throwing 60MPH velocity.
Combine that with measurables we've never seen before, it makes him an exciting prospect. With that ability to run, one that can be brought about slowly in the passing game as he evolves, like Jackson and Allen in the pro game.
Edit: This was posted just now on YT. It is a slow-frame side-by-side comparison of the throwing motions of Stroud and Richardson. AR-15 just as compact as Stroud—maybe a frame
quicker even. That's a great sign, although AR looks a bit too upright to me. I do like Stroud's mechanics a lot though, I must say.