Hmm, I didn't answer because I wasn't sure. And I still am not sure
That's to say, I agree with both your statements, and we do not want Twilight to add a token to Spring of Arda.
However, Twilight
is not a resource, but it can be
played as one.
And Spring of Arda currently states: "when a hero resource is
played..."
So one might assume that when a hero player plays Twilight during his turn he
is playing a hero resource.
It's all rather semantic, what's the point of making Twilight played as resource, since it already states it can be played at any time during any turn? You can counter it with Many Sorrows Befall?
Anyway, to avoid confusion I might add to Spring of Arda "(except Twilight)".
Stone-age did not end because man ran out of rocks.