Being strong makes a world of difference in any martial art, including Aikido. I never argued it was a substitute for technique, when combined with technique it is a force multiplier, so to speak.
Koshinage is absolutely dependent on strength if you want to throw someone who is not an uke and weighs a lot. Go try to throw around a 250lb judoka who is not playing a nice uke without having much strength. They are not going to charge at you and give your the momentum you need as a smaller person, you have to generate that momentum through strength/force. The same with any heavy attacker, they are just not going to give you much momentum.
I think the point you are trying to make is that without strength it will be difficult to throw a 250lb person who is actually trying to let you throw him.
Strength is not going to be what gives you the ability to throw a 250lb judoka who is resisting, unless you suck at judo.