stop it...

No it's not, you can battle anyone as long as you have one pokemon in your party. You don't need to have a starter on you to do that, and that guy has 165 hours played, so he just trained up his magikarp to lvl 100

If you watch the video you were given a link to, you would've seen that he took Squirtle as a starter, just because you can't choose Magikarp as a starter doesn't mean it's impossible to beat the elite four with it, as shown in that video