I found the answer here:
http://forum.xda-developers.com/showpost.php?p=30908422&postcount=186

Edit the updater-script, but don't try to fix it - remove the condition altogether since I know I'm on the correct device.  

Thanks adulfo!  

PS. bizarrely, after powering down, when I returned to recovery, another version (6.0.5.1) of recovery came up rather than 5.027.  Possibly x-Parts had improperly installed CWM, and the second install 5.027 made it possible to access it, because after installing with x-Parts I could not boot into recovery.  
Still, I don't know why CWM did not recognize the device as an ST18i (and thereby failed the script), especially since it had done so during the installation of CWM recovery.