I've never heard of such an instruction on the first roadblock of the Race. I had noticed something as a Note on the clue, but I hadn't had a chance to replay the episode and freeze-frame that to see what it said.
I haven't seen anything to explain that curious restriction either, and I'd rather not guess. Was there anything on the insider videos about this?
Edited to add: I haven't seen anyone mention this yet, but it appears the show is going to continue to not have a sign at the pit stop mat, nor likely to see traditional clue boxes based on this episode. It wouldn't surprise me if we don't even see one traditional clue box this time out.