Understood, but this is where our japanese linguist experts (also Aikido experts) may want to pop in.
The two techniques are implemented quite differently - so its kind of hard to see how the names can get swapped with these two techniques in particular.
Not being a lunguistic expert, but I might give some explanations.
Irimi nage means 'through by entry'
Kokyu nage means 'throw by breath power'
An as both techniques have both principles, the names can be switched easily. The names do not give specific hints on how the technique should be executed.
And in some variations iriminage and koyunage (aka sokumen iriminage) are done the same way but executed with the opposite arm as major contact with uke.
More confusion appears as kokyunage is not only the name for a specific technique in the examination syllabus, but also a collection of all throwing techniques which show that kokyu power
i.e. where you do not use gears and locks but just lead uke (dynamically) to fall by himself. And in addition you find some techniques called "ikkyu kokyunage" or "kotegaeshi kokyunage", which means that you do not focus on the joints, but on the kokyu power or even just means that you do not finish them with a pin, but with a throw or leaving uke free to escape by rolling.