I very much doubt China reverse engineered the S-300. It's quite possible they copied the vertical launch system, resulting in launcher units that look very similar. If they had effectively reverse engineered the S-300, they wouldn't have tried to buy the S-400 (an suggestion which was politely declined).It could be a HQ-9 Chinese reverse engineered S-300.
A link would be nice.I remember a military parade video of NK that shows NK has something that looks quite like the S-300, maybe an older version of the S-300PMU?