Opponents are able to play wild cards in standard format help, they are play prince renathal and such.
There are a number of cards which let you discover or summon cards from Wild in Standard.
What exactly is your issue?
Hey there, I came across this post looking for someone to have experienced the same issue: I just played against someone with a deck built with wild cards from Galakrond, The League of Explorers, Uldum and the like. Iām not sure if they really knew what was going on (if they were matchmade w/me by mistake) or if they exploited a bug.