If you want to find out savings from TimescaleDB's native compression, use this information view for the hypertable: https://docs.timescale.com/latest/api#timescaledb_information-compressed_chunk_stats
In the field, we're typically seeing seeing 90-98% storing savings across a large numbers of users.
For measuring ZFS, look at the numbers returned by zfs ...
Your DBA was kind of correct, for the typical scenario.
MOVE COMPRESS compresses existing data with Basic compression.
Any new data inserted with regular insert will be uncompressed. New data inserted with direct-path insert will be compressed.
If you want all new data to be compressed, then you can use OLTP compression. Then syntax for that is COMPRESS ...