If they slide in front of you and have position - it's a charge. But if they slide in front of you, then obviously they've left their man unguarded. So make the pass. Pretty simple.

Now - I think there's room to discuss what it looks like to be "set" and in position on defense. But simply saying that secondary defenders can't draw a charge is ridiculous and I tend to agree with the premise that you'd have the unintended consequence of players getting past their guy and diving out of control into whoever is there to get a call.