Yes, this is indeed very true and it's actually the best way in the game to get a Pokemon with its HA. And as for your second question, you don't need to unlock it or anything; as soon as you catch a Pokemon with it's HA you will have yourself the Pokemon just like you would have any other Pokemon with its normal ability. The only thing required is that you have to chain the SOS battles in order to increase the likelihood of getting a Pokemon with its HA, to appear which I think you already knew.
Hope I helped! :)))