Talking about -18DB, well could be that it was intended for analog workflows, I believe you when you say it.
What I do not agree on is that it is “BS”, in the digital domain you have the same problem with overlapping frequencies and the bandwidth of your summing bus, this is where gain staging will help enormously in my opinion and experience.
In practise I do not do gain staging at -18dBu but doing at levels in between -10 and -12.
However everyone has his own workflow with some common practises and a few specific ones
