I don't think that's true at all. Goeff Thompsons Dead or Alive: The Ultimate Self Protection Handbook is full of the SD skills. My wife thinks Martial Arts is a "load of bollocks" but she has used the skills in it that book to kill situations dead at the "interview" stage. I myself have used them on a number of occasions to stop incidents progressing to the "fight" stage.
You can train The Fence, and pre-emptive striking form The Fence, Threat & Awareness Evaluation (Coopers colour codes), Verbal De-Escalation, you can run training drills where you simulate arguments in bars and then afterwards discuss what happened that went well and what happened that went badly (i.e. you hit pre-emptively, but could you have walked away? or yo walked away and he jumped you should yo have hit pre-emptively etc etc) You can run drill where you go to a cash machine and a mugger comes p to you with a knife, and again dissect and analysis afterwards yo can practice keeping a loved one safe/getting them to safety, you can teach your students the relevant laws in their area and test them on them so they know in a criss what then can act and what they can /cant do (so that they are not worried about waiting until it's too late before they attack for fear of legal troubles). You can familiarise your students with the rituals of violence so that they know what to look out for, you can run drill with 2 or 3 or more attackers so you can practice positioning yourself so you don't get surrounded, practising taking out the first one or two guys and then verbally intimidating the third/rest into leaving, you can teach the skills to get back up off the ground as quickly as possible if you end up there (rather than staying there looking for locks etc). The Suzy Lamplaugh Trust has some fabulous information on keeping yourself safe at home, at work, travelling on trains/buses, out in public, on nights out, etc
The list goes on and on, but I'm boring myself now too

Point is, there are plenty of practical definable SD skills which can be practised and can produce results.