Xfinity is a company that I don't know the specifics about, but Xfinity WiFi appears as unprotected wifi but in order to actually use the Wifi for things, you have to go to website to pay to use Xfinity (on mobile devices you get a notification about this) and you can choose between just using Xfinity for a short time or have the ability to use it forever, so hotels often use it due to the ability to essentially rent the wifi. However, something such as a DS that can connect to the internet but can't use a browser will not be able to use Xfinity due to not being able to access the website (interestingly, the Switch is programmed to not connect to Xfinity because of this limitation). I think that when you connect, Xfinity sends blank data in all situations other than accessing the Xfinity website until you pay, and when you went to the GTS with Xfinity WiFi, Xfinity sent you garbage data that's supposed to cause an error in a browser, resulting in you getting a weird Pokemon with an undefined name (thus the nickname ?????) from the garbage data in exchange for whatever you had deposited in the GTS (even if you had nothing deposited at all).