I think that forum thread is confusing two types of metal roofing: large wavy S-shaped heavier-gauge corrugated steel (often galvalume), and flat thinner-gauge steel or aluminum with short ridges every 6"-8" that Matthias used.
As you said, it's also my understanding that the corrugated wavy stuff should be screwed on the ridges (that's how my friend did our sauna roof), not the valleys; and the flat stuff should be screwed on the flats, not the ridges. But I don't know whether nails have different recommendations (I imagine my friend would say don't use nails, use screws instead). I'm certainly not an expert, but my friend is.
I'd rather not contact Matthias though. I once wrote to him about some minor problem with his website (many years ago, way before his YouTube channel was popular), and his reply was rather mean-spirited and completely uncalled for. I don't hold it against him, but it's soured my opinion of him ever since. In any case, once you make holes in the metal, you can't really remove the screws.
As you said, it's also my understanding that the corrugated wavy stuff should be screwed on the ridges (that's how my friend did our sauna roof), not the valleys; and the flat stuff should be screwed on the flats, not the ridges. But I don't know whether nails have different recommendations (I imagine my friend would say don't use nails, use screws instead). I'm certainly not an expert, but my friend is.
I'd rather not contact Matthias though. I once wrote to him about some minor problem with his website (many years ago, way before his YouTube channel was popular), and his reply was rather mean-spirited and completely uncalled for. I don't hold it against him, but it's soured my opinion of him ever since. In any case, once you make holes in the metal, you can't really remove the screws.