I know some people ( just a few ) just use open sights on HFT courses. Maybe they've sneaked under the radar but I don't think the rule was to make people exclusively use telescopic sights ... just not telescopic sights plus lasers.
The Pete's have often said that they try and keep the written rules to a minmum and hope that common sense fills in the gaps. Otherwise you would have a set of rules 20 pages deep.
One of them will answer this.