interface MongoClientOptions in mongodb.d.ts contains property definition as below:

/** Comma-delimited string of compressors to enable network compression for communication between this client and a mongod/mongos instance. */
compressors?: CompressorName[];

You can already see the issue from the discrepancy between the documentation comment and field type. Documentation is in fact correct here and "compressors" property is a comma delimited string, not as the type suggests an array of strings.