That's a myth littered on the internet by some dumbass people promote closeness of Japanese & Korean in all imaginable ways.
Glad I dont believe everything I see on the net ... I was actually told this by my Japanese teacher at University. And I'm not saying it easy - hell learning Japanese isnt easy, BUT once you've learnt all the nuances with the grammar (my least favourite point of Japanese and the part I'm most likely to fail on forever) you can apparently learn Korean which is similar in grammatical nature to Japanese easier.
It's on the same line as learning any language which latin/greek roots - When I, as a native english speaker with only 6 months experience learning Spanish, listen to some European languages I get a rough idea of what they're talking about because the words are similar because they all descended from the same places.