There is a difference between a short SIMPLIFIED answer versus a longer more detailed answer that may require a LOT of new thinking to understand.
Sounds to me like you were given the (usual) simplified, sound-bite answer.
Try to get this:
Satanism is about DOUBT and not CERTAINTY.
If you want CERTAINTY about afterlife suggestions you have zillions of faith-based religions out there who will assure you and those who come to your memorial service that they KNOW there is an afterlife.
If that is your cup of tea then have at it. You will be surrounded and assured by BILLIONS of other true believers.
But here we simply say it is wiser to not PRETEND you are CERTAIN about what happens after you are dead since... (Drum roll please!)...
You are not dead yet!
Satanism says DOUBT will set you free from the "certainties" of faith.
Faith is, after all, believing something is true just because you do.
Satanists do not BELIEVE in an afterlife.
That's because unless and until YOU are really, really dead (and stay that way!) you cannot KNOW if there really is an afterlife or not.
You can SUPPOSE there isn't.
You can SUPPOSE there might be.
But like virginity, you either are dead or you are not!
Evidence can lead you to support your ASSUMPTIONS but this is an area that could fool you ...in the end!
So NOW you have TWO agendas:
1. SATAN as a real guy. (NOT part of Satanism).
2. BELIEF in an afterlife. (NOT doubt since you lack personal validation you silly, still-breathing person you!
Maybe as you keep digging in your gradually-surfacing and increasing pile of issues you really need to be willing to question (DOUBT!!!) all these BELIEFS you have before you think you are a Satanist.
Satanism is not just mindlessly screaming "Hail Satan!" to antagonize people who think you are kooky.
Satanism requires study.
Increasingly it sounds to me that you first need to do A LOT MORE STUDY!