It's because there are two types of skill involved in playing League: mechanical skill and strategic skill.
A champion can require a lot of one and almost none of the other, so people who value the opposite one will consider them to be a no-skill champion.
As someone who favours strategy over raw APM / mechanics, I'd consider a champion who can use their mechanical skill to get out of huge strategic mistakes to be a "no-skill champion". For example, I have very little respect for someone playing Lee Sin, because while he is mechanically complex, he has so many tools in his kit to escape bad situations that he needs fairly little strategic skill.
That's certainly not an opinion shared by everyone, however. The other side of the coin are people who for example would consider a champion like Evelynn no-skill - she has no skillshots or complex mechanics, but needs a lot of strategic skill because she is so fragile and such a bad duellist that she will die if she makes the slightest mistake.
But the above is more about champions like your aforementioned Yasuo, or Lee Sin, or a few others.
Riven is a borderline case anyway. The only real argument is that her high number of dashes lets her get out of situations that she probably shouldn't a little too easily.
She needs a lot of mechanical skill, but she is still very fragile and needs a decent amount of strategic skill too.