It's well explained here. I don't think there's a difference between the two as long as time evolution is concerned, meaning in your examples, you can use both 〜につれて and 〜にしたがって. An important difference is 〜にしたがって can also mean "based on" or "according to" (as defined in 意味② on the page above), e.g. ルールにしたがって (= following the rules). If this is not what they meant with "一方方向の変化でなくてもでなくても使える", I don't know what they wanted to say.

It's well explained here. I don't think there's a difference between the two as long as time evolution is concerned, meaning in your examples, you can use both 〜につれて and 〜にしたがって. An important difference is 〜にしたがって can also mean "based on" or "according to" (as defined in 意味② on the page above), e.g. ルールにしたがって (= following the rules). If this is not what they meant with "一方方向の変化でなくても使える", I don't know what they wanted to say.