Maybe the first is why you think the second is always true? Where I live if I just leave laundry on a drying rack it won't dry for days, and once it is dryish it already smells stale and mouldy.
I'm glad that it works for you, but maybe the advice differs based on location, climate, maybe even architectural choices.
They started out right and acknowledged that things can be different in different climates. But then in their second paragraph they made a categorical statement: “Still, drying clothes indoors has no ill effects, I have no idea where you got this.”
That is a general statement. They don’t think drying clothes indoors can have any ill effects. Ever. At any humidity level.
So they realise that the drying time can change quantitatively. What they miss is that there are qualitative effects which can happen too. And in their hurry they dismiss other people’s lived experience.
Still, drying clothes indoors has no ill effects, I have no idea where you got this. It's definitely better than exposing them to high temperatures like in most apartment tumble driers.