The method, CSVLayer.setDefinitionExpression(), does not filter data as defined in the definition expression and loads all the features in the map.
Bug ID Number
August 27, 2015
December 1, 2020
Operating System Version
2008 64 Bit
After review by the development team, it has been determined that this issue is related to a known limitation with the software that lies outside of Esri's control. The issue's Additional Information section may contain further explanation.
CSVLayers do not support queries that need to be run on the server (i.e. where clause based queries). This is noted in the doc.