Yeah, they are currently decommissioning all West German nuclear plants (initially announced by the Social-Democrat/Green government before 2005, then delayed by the Christian-Democrat/Social-Democrat government, then finally set to 2022 after Fukushima), but this is much later than 1990, when the Greifswald plant was decommissioned. Of course, that was only 4 years after Chernobyl, that may help explain the decision...

Both Greifswald and Rheinsberg were built according to German standards. East Germany even produced the reactor pressure vessels themselves.

Also, the blocks 5-8 in Greifswald were modern VVER-440/213 which are far more reliable and safe than for example the SWR-69 reactor types used in Neckerwestheim I, Kruemmel and Isar 1 which were all in operation until 2011.