I'm not sure I can agree with the above statement, on any level. Why is it the most important? why is it the most difficult? Why does it rely purely on timing?
It is difficult for us all to comprehend aikido and as such, we can discuss it endlessly here on this site. Personally I like ikkyo, I find it really easy, of course timing is important, depending on the dynamics of the attack, but purely? surely not, there is so much more that has to be in place for the timing to be effective (distance, balance, structure, relaxation, mind, intent etc)
I don't see any of the aikido techniques/exercises as having a hierarchy of importance, they are all useful tools for practicing the principles of aikido, which I see as the raison d'etre of aikido.
I am not saying that you are wrong, as I would be interested in why you say what you say.
I don't fully understand ikkyo myself but just from experience and teachings of high ranking aikidoka, it is not the technique itself that is important with ikkyo but how to read intent of your partner and act on it accordingly. The principle being you are already there even before the attack has begun, but not too early. It is more the mental aspect that you are trying to hone much more than the physical. ikkyo from suwari waza is a good example of this. You dont wait for the attack to start. Looking it at this perspective, ikkyo is not that easy...It maybe easy for you but not for me.
I imagine if you are faced with an accomplished swordsman and we try ikkyo as a defense, you will get cut down. Forget about the youtube videos where a lot of aikidoka successfully use ikkyo for defense. Some of them might be legit but some not so imo. Remember you only need the farthest 3rd of the sword to kill. Only by being "ahead" of the opponent can you successfully defend. This I think can only be done by reading the person's intent. By saying that timing is not an important factor for ikkyo I disagree because ikkyo is counterintuitive...you do strong irimi directly to an attacking opponent. Be too early, you will get cut, Hesitate with the irimi and you will also get cut.
I would think Osensei put ikkyo (first teaching) as first since it is important enough to warrant it being the first to be taught?
This is just my understanding though.