Regardless of the case of the microSD folder [poi, POI, Poi etc], the folder will be named Poi when you transfer any .gpi files from it to the unit. That folder will be created exclusively in the root of the unit memory using that method. Again regardless of the case of the folder name in update.txt and the case of folder name in the unit, the command will be executed successfully [i.e. POi will find poI, Poi, POI, pOi etc and vice versa]. However, the gpi file name must be exact and wildcard '*' will not work as you found.

Prior to you posting the edit above, using a NA 1300 i did some tests. This is the update.log result for various commands where the folder Poi was originally created by accepting the nuvi's request to transfer gpi files from the microSD's POI folder:

nüvi 13xx Series Software Version 6.20, UID: 3xxxxxxxxx, HWID: 3xxxxxxxxx
------------------------------------------------------------------------------
Parsing "del,0:/Poi/AUS-Airports_Jan12.gpi"
Success
Parsing "del,0:/POI/Redlight Speed Cameras.gpi"
Success
Parsing "del,0:/Poi/*.gpi"
Error
Parsing "del,0:/poi/SCDB_Oceania.gpi"
Success
Parsing "del,0:/pOi/Schools ACT.gpi"
Success
Parsing "del,0:/poI/Schools NSW.gpi"
Success
Parsing "reboot"

So if you still have the original files on the microSD it's easy to choose the correct name to delete them. If not then it's blind man's bluff/shooting in the dark.

A little bit of [maybe] interesting trivia. As well as the case of the poi folder not being of any concern to the unit, the actual naming of it doesn't matter to it. You could name it 'xxxx' and dump it in the Garmin folder or .System folder and the unit can still find and read the files.