I think the guidance is reasonably clear. Since they quote SQM, that’s what lighting designers would use, not SQM-L.
I’m OK with zoning in principle, though I think they underestimate the bad effects of poorly designed and implemented lighting. I don’t really have a problem with Weymouth Town Centre having light levels of 25lx before curfew – it’s busy and you need to be able to see well. The trouble is that 25lx at street level is often accompanied by vast amounts of light going upwards and outwards. This means that 10km away in the national landscape there is a gigantic light dome filling about half the sky and at least as bright as a local floodlight.
The real issue is that planning authorities more or less ignore even vague guidance like this. I had correspondence with the New Forest National Park Authority a few years ago when they were consulting on their planning guidelines and managed not to include ANY guidance on lighting (They did add a couple of lines saying that exterior and interior lighting should not be obtrusive). Even then it felt like trying to bolt the stable door after the horse had bolted as the village I then lived in was full of uncurtained skylights, security floodlights and the like. I was particularly annoyed by car parks that had carefully designed downlighting (which worked well) but someone in the pub or similar building would retro-fit an enormous security light and it would take years for the National Park Authority to respond. In the Dorset National Landscape there are numerous examples of such things as floodlighting for equestrian or agricultural purposes which is very poorly designed and which ruins the dark for miles around and the Council is simply not willing to act unless it is shining straight into a residential window.