In theory RDAP is open to all (federal) offenders. In practice, only those with drug-related offenses make it into the program, and the wait list is years-long and most eligible inmates complete their sentence before their spot on the wait list comes up. The reduction of sentence is applied against the drug-related sentence. The prisoner is moved to a halfway house up to 1-year before the original expiration of their sentence, and can be returned to prison for violating the rules of their transitional release.

Yeah but these are pretty common charges no? Seems relevant. For instance, my understanding is in some instances if you have a drug disorder with possession you're in a better position than merely found in possession and don't even use drugs (or use them without having a "disorder")

Yes, they're quite common. But pure drug offenses (other than dealing) generally get sent straight to diversion if they qualify (and space is available). Most drug offenses in addition to other crimes and this generally makes the defendant ineligible for diversion. Given two similar defendants whose only difference is that one is substance dependent and the other is not, the dependent defendant will never get a meaningfully shorter sentence than the defendant who is not and is more likely to end up with additional restrictions on their parole that make it easier to send them back to prison.

must be a lot of attorneys lying to their clients.

The programs are not memes, but your understanding of them, and how they are actually discussed by attorneys with their clients, appears to be based on internet memes of these programs.