I do not know what was on the controller's mind, obviously. I can't tell whether he misinterpreted the altitude difference, whether he computed the cleared target altitude FL200 vs. actual altitude of FR-9431 (which was 800 feet), ... All I can say that at the time when he reported the 737 descending 800 feet below FR-9431 was at FL192 and EI-433 was at FL210 subsequently levelling off at FL210 according to Mode-S.
From the Mode-S data it is clear that separation with the clearances in effect was not assured (in particular as EI-433 descended much faster than FR-9431), however, an actual loss of separation had not occurred.
The instructions were clearly issued in time to not permit the situation develop into a loss of separation and recover into a scenario where separation was assured again. I am not saying anything against the controller, his plan obviously didn't work out as he intended - I am just upset with what was made out of this by a couple of media (based on the headline of the video).