I think Mr Wayte is playing it safe. If he advises changing pressures and a problem occurs, he could get the backlash. Although the carcass may be similar in construction, I'm pretty sure it won't be the same materials used from the 60s when tyres were akin to rubber blocks an had a much firmer structure altogether. Ultimately, it's down to the rider and whether one uses one's own judgement or that of another authority (I'm the former, not the latter). As an aside, when I first started riding the Flash, I used BSA's advice on pressures on Mitas tyres. The bike was a little 'wallowy' at times but I put that down to it being an old bike. When I mentioned this to my engineer pal (50 years BSA/classics experience), his eyes almost popped out and gave the reasons mentioned here - he also raced on IoM.
I guess if you're pottering around, then pressures may be less of an issue than Berger's or Musky's (heh). Wonder how the cops would ascertain the best pressures for our bikes if some arsy kid with God complex pulled one over and decided he was going to be clever?