A Supreme counter is a Pokemon that makes the best performance in all scenarios. I'm also really skeptical about Feraligatr as a supreme counter, for a few reasons:
1.- It gets blown into pieces with Groudon's Solarbeam.
2.- Against Earthquake Groudon's, Gyarados it's SO much better.
Maybe they're lazy while writing the guide?
Go and max out that beautiful Magikarp. I have 3 Maxed out Gyarados, and they're just awesome.