[AtlUmpSteve]

What you say is logical; they have a legal substitution available, that the FLEX can enter for the DP. And, it now complies with the shorthanded rule, ASA 4-1.D(2)a, which says "If a team begins play with the required number of players as listed, that team may continue a game with one less player than is currently in the batting order whenever a player leaves the game for any reason other than ejection."

So, they started with a legal number, were batting 9, can still bat 9.

The reason I say this is that this was an editorial change made by ASA in 2006 or 2007 (can't find a 2006 handy). It used to say (in 2005 and previous years) "currently in the lineup"; that meant you could not legally drop to 9 players when previously playing with DP/FLEX as a result of an ejection, unless you had a legal substitute (which you chose NOT to use).

Over the years, I had this discussion with several of the upper hierarchy in the NUS; they wanted the forfeit if a player in the game was ejected and there was no legal sub on the bench. Henry argued that the team shouldn't be treated differently if the player who earned an ejection was the DP or the FLEX, rather than a position player. I argued that a team that had a sub they weren't going to use shouldn't have an additional move available that a team without a sub wouldn't have (but has under any other circumstance). When the new broom swept, someone agreed with me, and made that small change.

Edited to add: Found my 2006, still said "lineup". So the change was made after 2006, with the 2007 book.

ps: Checked ISF while posting other rules; would be a forfeit there, cannot drop to 9 by an ejection if playing with DP.