Full Rules for targeting a creature:

The target of a spell must be within the spell's range. For a spell like magic missile, the target is a creature. For a spell like fireball, the target is the point in space where the ball of fire erupts.

Most spells have ranges expressed in feet. Some spells can target only a creature (including you) that you touch. Other spells, such as the shieldspell, affect only you. These spells have a range of self.

Spells that create cones or lines of effect that originate from you also have a range of self, indicating that the origin point of the spell's effect must be you (see “Areas of Effect” later in the this chapter).

(from later in the section)

A typical spell requires you to pick one or more targets to be affected by the spell's magic. A spell's description tells you whether the spell targets creatures, objects, or a point of origin for an area of effect (described below).

The rules of a specific spell may override this (like Fireball does when calling affected creatures targets) but if the specific spell doesn’t, these are the rules that apply.  Ice Knife follows this rule, and the clear distinction between the target and other creatures indicates (at least to me) that the spell is intended to only have one target.

I think that actually, fireball might be the only spell that calls creatures in the AoE "targets" (if not, then only one of a small number), and therefore might be the oddball out. Under either interpretation, it still wouldn't be twin-able.

But, your interpretation is fine, especially because there is only one spell different. I also think that it is still perfectly reasonable to eliminate all area of effect spells.