I've never indicated making that assumption though. Ive discussed your response to a robber who you judge to be unskilled.
As I mentioned in the first post I indicated this is usually obvious (if they are skilled/unskilled) and if you are trained you will recognize this.
The easiest pointer is how they hold the knife - if they havge it out in front of them (pointed at you) they are probably unskilled. Skilled handlers will hold the knife at their side a few inches from their body pointed away from you at a 30 degree angle.
Unskilled handlers will also glance at the knife a lot (you can use this - time your attack to their glancing).
If the robber is skilled then, clearly, you need to make a different assessment
(and I guess then it comes down to "is he planning to attack me")