So where's the question?
In the case of the Sigberg usage of Sica Drivers, I know that Thorbjørn had some assistance in selecting Drivers from an experienced hand. Following his threads, you can see how he has worked through iterations of tuning the active XO and optimizing the Driver to give what most will agree is a first rate FR. I don't know how much help he continues to have in his design and testing practice beyond his consulting with somebody to help select Drivers. Regardless, he's put two Speakers to market and is working on a third!
In terms of the raw Coax itself, admittedly, I like the looks of the FR sweeps Sica shows for the Drivers. I think every Coax I've looked at is pretty rough past around 8-10K. For my level of experience, that is off-putting. Seeing someone wrangle that into something not-just respectable but actually marketable as a high end Speaker is pretty cool. Granted, he was starting out with a good performing Driver which as I described seems better than most.
The Amateur Designer I mentioned has been building and designing Speakers for some 60yrs and I think he said he built his first Speaker at 14 or 16. *shrugs Like several Pro Designers I've spoken to, he places a high value on Driver selection and matching for the purpose he is designing for. His motivation for his designs is not something I can Speak towards, though I've seen measurements of final projects and they are impressive, including use of the Seas Coaxial. (I don't know the specific model, but I'm certain it is not the one with the clear cone.)
Beyond that, he does his own passive XO work to the best of my knowledge and is not using any active DSP solution in his designs. Sigberg is using the Hypex Fusion Amps.
For anybody building a kit or whatnot, I think the biggest distinction is in the field of how much time you are going to spend tuning it, and how much you will spend on components to fix problems in FR. Not all kits are created equal, and some only take their XO design far enough to be passable without overwhelming the DIYer with 10-component Tweeter circuits and the resulting cost.
I think it's been shown that many instances of "poor design" stem from simply making something good enough as opposed to doing it right. Doing it right might mean extra work or more stringent control in matching Drivers as well as better XO design for a passive Speaker. In that manner, it's a give and take because in the end you are negotiating compromises and balancing cost. That is something any strict DIYer, kit designer or commercial manufacturer is actively doing all the time.