From the same place as you. Teacher. Then plus common sense. In order to blend with, thus Aikido.
Defined as passing behind.
Now if you also want to use it as passing straight through then so be it but in Aikido that would lead to confusions in communication and people doing irimi straight into a spear.
I don't believe you or anyone does move straight into anyway unless you're hitting someone with your chest or belly. Or maybe a head butt.
Believe what you like. We explicitly aim on shomenuchi to not move off the centerline but to meet the strike on centerline, deflecting it just enough to make it safe for nage to then slide in to position for technique - as opposed to letting uke have centerline while nage darts or slips past it. Our goal is a straight centerline entry.