This kind of trick has been around for ages and I'm frankly surprised that WN
got caught by it, however briefly it may have been used to move up the boarding queue.
Initially, this trick was used against unsuspecting web sites that were storing the price of the items you were buying online as part of the web page itself. You'd save a page on your hard drive, change the price of your item and click on the Continue or Next (or whatever) button and upload your new price to the web site, and you'd be the proud owner of an MP3 player (there was no iPod back then) for $5.00. Now, all prices are stored in a database and web pages just contain references to the database instead.
Southwest can easily fix this, by making the boarding group letter a graphic image, rather than an actual character, for instance, and changing the web page code to force the image to be loaded from their web server. Not fool proof, again, but much more complicated to do at home.
Recep Tayyip Erdoğan has no clothes.