Shiny pokémon can be caught without the pokéradar, although they are very rare.
Think about it this way, the pokeradar has only been around in dp all the shinies in r/s/e can still be legit too you know ^-^;
I have 2 shinies that I've seen (caught1) and it wasn't with the pokeradar because I am a MEGA failure with it XD and my shinies are still legit
That guy was lucky if his gyrados is legit.