A diabolist is a term that the Black Flame Immersion host uses to define his path in the Left Hand Path. This is how I use the terminology here as well.
The reason for using this term, was in the loaded terminology of Luciferian, Satanist and so on.
A Luciferian derives their identity (as does the Satanist) from the context of the Christian faith. As the host of the Black Flame Immersion views this as the Abrahamic Parasite, a distinction is sought.
Another concern is over the materialistic (i.e. atheist) view of many Satanic groups, such as The Satanic Temple and the Church of Satan. For these groups the word Satan is used as an image or icon of the individual self.
The Church of Satan has no real interest in a theistic approach to life, nor any concept of Becoming something beyond the body.
As for The Satanic Temple (which I know little about), their role seems to be politically motivated – choosing an icon to infuriate political right-wing Christians.
Using the term diabolist can be very freeing for some, as it doesn’t hold one to the loaded terms described above which come with their own baggage.
For more information on this, consider listening to the Black Flame Immersion podcast: