I just got a payment e-mail from ISC for a shop done on March 3. So, it was paid earlier than I expected, and sooner than the 5 to 6 weeks that I've seen in the past. My February shop with them was paid in 5 weeks, and the March one in just a little more than 4 weeks.
It sounds as if they are telling you correctly what their payment time frame is. Not sure when they've ever paid more quickly, but in the two-plus years I've been shopping for them, my payments have always been within the stated 6-week period. I'm going by the overall shopper information document I have.
Maybe the information in your shop guidelines was incorrect? If so, they should certainly correct it!
I learn something new every day, but not everyday!
I've learned to never trust spell-check or my phone's auto-fill feature.