Confused about keeping quiet

There is a hadith, which says:

" “Whoever committed any of these abominable acts but was covered by Allah, let him or her not expose what Allah has covered.”

There are some parts of this that I don't understand.

If a man/woman has committed a sin, lets say adultery. And they make their toba.. but they don't get the punishment because they are told to keep quiet about it. Then why do we have an existing punishment?

And when someone says, 'best they get the punishment (stoning) in life rather than get it in hell' doesn't that contradict? I mean, if you keep quiet about it.. you won't get punished, therefore it awaits you in the afterlife right?

Another thing, is it really true that if you are punished in life you will not be in hell?