As long as they can prevent the changing of their behavior, they can keep their gambling activity in secret and no one will knows about that. They can playing gambling without any one knows and disturb them and they will not breaks their limits because that's the important things for them. They can keep their gambling activity secret but they will also needs to learn about self-controls so their gambling activity will not makes them lose much money. But many people can't realizes about that and willing to still playing gambling without control and that makes them becomes addicted to gambling. If they becomes addicted to gambling, that will be difficult for them because their family will knows that from the changing of their behavior and will asks them about what's going on with them.
If that's the situation I can agree, if you can seperate things then that's okay keeping your gambling as secret without any behavioral changes will give peace in you. It's easy to keep your gambling secret but when you exceed or over spend your time, addiction can be trigger and that's going to create a bigger problem, gambling is for sure an addictive activities where you need good self-control and you need to make sure that you have that capabilities to stop or pause when the time you needed to stop.
In most cases, those who choose to keep their gambling as secret as possible are people who don't want any argument or they are not open to any opinions about gambling that's why they want to engage and they just want to play as discreet as possible.
The worry in this situation is when they turn out to be unable to resist all the temptations that exist in gambling that can make them tempted and fall into unconsciousness so that in the end they experience a significant loss of money which in turn the inability to accept the fact of losing can make them experience unconscious behavioral changes and this can be felt by those closest to them such as their family members. If for example the situation they can maintain their involvement well then yes maybe everything will be able to go well, but still the name of the possibility of experiencing changes is very likely to occur such as entering the addiction phase without realizing it, and all of this is very possible because as you said that gambling can always make someone tempted and addicted.
Yes, it makes sense that it could be a decision to keep their gambling activities secret because they don't want many people to interfere with their gambling habits, they may be uncomfortable with some opinions that can interfere with their involvement in gambling, but still the point is that they must really be able to maintain, manage, control and limit their gambling very well, because when no one knows your bad habits then you will not seek help from others if one day something happens to you caused by gambling.