As you noted - when you delete data you're going to still have the same size in dba_segments, that space is just free for reuse.
Some modules have specific notes on MyOracleSupport on purging data, however it's a bit of a dark art - as you'll no doubt be aware, data is so intertwined in EBS that it becomes very difficult to purge a specific time period accurately and consistently.
I am not 100% sure I agree. I found IBE_CT_RELATED_ITEMS table was consuming 1.5 TB of space in dba_segments, used Note 753551.1 to purge it and immediately saw a decrease in dba_segments and tablespaces for APPS_TS_TX_DATA and APPS_TS_TX_IDX all of a sudden had 1.5 TB more free. Still could not shrink datafiles due to high watermark, but at least had room for more growth.