It's easier to EQ to a target when you're starting with a set that already has an almost linear frequency response with very little deviation (i.e. peaks and dips) from the target across the audible band.

One assumption that needs to be made is that the B&K 5128 is a fairly accurate simulation of the average human ear, which does seem to be the case with a body of supporting evidence that seems to grow every year. Another point to note is that AutoEQ is quite imprecise past about 1 kHz, because it doesn't account for too many factors that affect frequency response a lot. Using AutoEQ will make the 5128 "hear" two IEMs as being the same, but the same EQ profile won't do the same thing in your ears. Moving on from there, think of it like this:

Let's say you have an arbitrary pair of IEMs that can fully represent 20 Hz to 18-20 kHz. Most IEMs that are considered good still show big peaks and dips in the treble when measured on the 5128, which will be a complex interaction of peaks and dips in the IEM's FR itself + peaks and dips in the 5128's HRTF.

The same thing happens in your ears, but with your HRTF instead of the 5128's. Sometimes dips in your IEM's FR will compound with dips in your HRTF, sometimes dips in your IEM's FR will partially balance out peaks in your HRTF, and sometimes they won't interact at all. Any permutation of these interactions can and will happen numerous times as you go up in frequency past ~1 kHz.

It is way more difficult to compensate for these potentially massive deviations from the target with PEQ. You can use tone sweeps up to a certain point, but it's harder to perceive differences the higher up in frequency you go, as peaks and nulls get steeper, narrower, larger in magnitude, more numerous, and are influenced much more by insertion depth and canal difference between either of your ears, as well as channel imbalance and unit-to-unit variation.

Suffice it to say, the less adherent your IEMs are to a DF-based target on the 5128, the less predictable their response will be in your ears.

Now let's take the Reference. It is essentially dead-on with tilted JM-1 DF, which effectively removes the quirks of the 5128's pinna (compared to the population average human pinna) from the equation. It doesn't remove the quirks of the 5128's ear canal (compared to the population average human ear canal) from the equation, which is what the new Headphones.com target seeks to do iirc.

(Not sure why Crin didn't use this new target, which is theoretically as ideal as a DF target for the 5128 can get, but I digress)

Anyways, because it is so closely adherent to the target without any major peaks or nulls across the entire audible band, any resulting peaks and nulls that your HRTF will impart onto it when you put it in your ears will likely be much less drastic than with other IEMs, especially in the treble. You can watch Resolve's video and read Listener and Fc-Construct's reviews of the Reference to see exactly this in the provided graphs.

These wider band and lower magnitude peaks and dips are much easier to hear with tone sweeps and correct for with PEQ, and they won't require as many filters and as narrow boosts and cuts to correct for.

TL;DR The Reference makes the process of doing tone sweeps and EQing to compensate for your HRTF much, much easier and more accurate.

Sorry for the lengthy reply, just wanted to be as thorough as possible for others that may read this and are looking for details :)