You did not hear him correctly. You need to read Side Letter 9, Paragraph H and Q&A #37 and #38 again. An appendix to Side Letter 9 listed 851 AirTran Captains that were to be given Captain Retention Slots that expired on 9/27/2020. No SWA FO could upgrade until all AirTran pilots with a Captain Retention slot sitting in the right seat were offered their seat back.
There were 3 conditions that would cause a Captain Retention Slot to be surrendered forever (SL9 Para H.2 through H.4):
1. A pilot retires
2. A pilot voluntarily downgrades himself
3. A pilot with a Captain Retention Slot and sitting in the right seat bypasses his first opportunity at re-upgrade.
Let's run through an example calling it Vacancy Bid 14-A (the first vacancy bid of 2014). Let's say Vacancy Bid 14-A had a plus 15 B737 CA and minus 25 B717 CA for a net reduction of 10 CA positions (as 3 B737-800s replace 5 B717s). Since normal Southwest Captain protections apply (SL9 Para H.1) in addition to the Captain Retention slot program (SL9 Para H.2), 15 of the displaced AirTran CAs would be guaranteed B737 CA vacancies if they wanted them (Q&A #37). The other 10 B717 CAs would be involuntarily downgraded. Since the downgraded AirTran pilots had Captain Retention slots, they had to be offered re-upgrade prior to any SWA FOs upgrading regardless of system seniority.
When referring to fences coming down, the fence between fleets dropped with a net reduction of CAs (for that one month's bid). In the B717 early departure scenario, the reductions would come from the B717 side triggering the movements listed above. If it was a seasonal downgrade and B737 CA positions were reduced as well, SWA B737 CA could displace to B717 CA (and thus force junior AirTran B717 CAs to the right seat). However, those downgraded AirTran B717 CAs would not lose their Captain Retention slots.