Yep, gangster is right, I just checked it as well.
I can think of two possible explanations that wouldn't make this a goof:
(1) David messed up, didn't know his facts;
(2) The Narn and the Drazi were kicked out of the Alliance after their attack on the Centauri and only recently readmitted.
I doubt that (2) applies though. It wasn't mentioned in the series - I suppose there might have been some punishment (forcing them to pay, limiting their part in the Alliance, something like that) but I doubt they'd have been kicked out so that it was never mentioned. The Drazi, maybe, but I really doubt they'd have kicked out the Narn and the topic never even coming out between the rest of the top people and G'Kar, or in G'Kar's last message to Ta'Lon.
The third explanation is, of course, that this was a goof. Maybe JMS meant to say Rangers but typed the Alliance and didn't notice it? Maybe he forgot? Maybe he thought using "the Alliance" would sound better to the potential new viewers?