If the belief in satan is meant to be theistic
If.
A major part of people calling themselves Satanists are not theistic.
And while I don't know whether atheistic or theistic Satanism is older (I would assume the latter), it was first made really popular in more or less atheistic form.
I wouldn't say religion needs a deity, as there are several established religions that have no deity, or at least not something they consider a deity themselves (e.g. Taoism and forms of Buddhism).
One could say that someone who does nothing even remotely religious with their Satanism (i.e. no rituals, no symbolism, no religious feelings, no celebrations, ...) and considers for example the Satanic Bible nothing but their favourite self-help book, might rather be considered to have a philosophy and no religion.
But that is not what being an atheistic Satanist has to mean.
Yes, sometimes being an atheistic Satanist rather seems like belonging to a fandom - but in these times where chaos magick spreads, can we really see a clear distinction anymore between having a deity and being fan of a fictional character?
Theism refers to "belief in a deity" in a general sense. Deism is a form of theism where the deities revered are unnamed, fluid forces as opposed to concrete beings.
I agree that deism is a form of theism as it is a believe in a deity.
But as far as I know, deism means believing that a deity created the world and then stopped interferring with it.
Might be that there are other definitions of deism, though, that are a bit closer to pantheism, i.e. there is a deity but it doesn't contradict natural laws but rather manifests through them.
That would be probably what people mean when they say that LaVey was a deist.