This is possible because the more they are closer to their gadget, the more they feel that urge to open the app and gamble. This is not the same thing as a gambler who gambles often offline, that alone will limit him automatically.
It's true but it can only work if the person gambling offline doesn't have a land-based casino or a betting shop near them and they need to travel from their place to somewhere far away to gamble which they might not always do because you wouldn't want to travel more than 20km just to gamble with $20 or something which a person wouldn't bother doing if they are gambling online since they can do it from the comfort of their home, sitting at the couch having a snack.
However, if the person has a casino or a betting shop within walking distance from where they live, it might not have that big of an effect on them and their gambling addiction because whenever they have some money, they can walk into the casino or the betting shop and gamble it away without any problems or a second thought.