It says it uses Euler integration, what do you expect? At least the really bad interactions are mostly handled by absorbing the planets into one another, even though not all of them are.
Nice observation. The inaccuracy is greatest when masses are near, so absorbing close masses limits how near they can get. When I wrote one of these simulators, I had masses zinging off everywhere when they "collided".
Question: does this inaccuracy systemically affect the development of a "solar system" from a "proto disk"? I think, by increasing randomness, it would actually tend to work against it somewhat.
Also, the odd effect of the proto disk expanding after the sun forms is caused (I think) by masses falling into the sun, and thus further away from the rim of the disk. This greater distances reduces the gravitational force on the rim, and so their velocity ("centrifical" force) can carry them away.
