Fungibility of any coin (btc included) is only at stake if some coins are worth less than others. There is currently a hudge hype about anon crypto because of this. The story goes that if a certain coin was used in a nefarious or illegal way it is worth less since some will not accept it. THIS HAS NOT HAPPENED YET. At least not on a large scale as far as I know. Anyone please feel free to enlighten me.
It did happen on ETC. Exchanges didn't accept (well, did accept and then froze) the coins that came from the "Robin hood group" that emptied the DAO to "protect it from evil hackers", when they tried to trade them.
So these etc coins didn't have the same value or got the same treatment as other ETC coins.