I've often observed a forefront of theory, taking risks and doing great innovation. Professional designers are typically interested, knowledgeable, and admiring of those in that forefront. They look for ways to integrate what they're learning into their own designs. Making neo-trad in a way quite foreseeable. Usual design moves include focus on accessibility and production quality.
Harenstam's comments fit the above. Free League use their Year Zero Engine for other games like Forbidden Lands, which I haven't played but take to lean into sandbox with an OSR feel. Editions of D&D draw on successful experiments around them. Notably 4e, and I can see it in 5e, too. They're always limited by needing to make the game open to all comers. A commercially motivated conservativism.
EDIT Out of curiousity I dug up the original article. Here is a fuller quote (Harenstam)
And later on in the piece
But NONE of that tells me much of anything about how Mutant Year Zero plays. I mean, I have no experience or point of contact with the game, so I have zero opinion on it. The term 'neo-trad' however is almost useless to me. Over the past 40 year that style of game (though the label is newer) has existed, but they're all over the map in terms of important game play factors. Frankly I don't even see it as really being a coherent categorization. Yes, there's some vague generalizations you can make, but in fact if you were to subject these games to analysis in terms of Baker or Edwards' you would learn vastly more. All I learned from 'straddling the line between Gamma World and Apocalypse World' is that it pro ably (as if the name didn't convey this) takes place in a post-apocalyptic milieu. Is the focus of play on reproducing some aspect of post-apocalyptica? On survivalism? On environmental exploration? Tactical combat challenges ala Aftermath? What? You're invoking 'neo-trad', but all that tells me is there's likely more of a focus on player-driven characterization than, say, Gygaxian player challenge. That's SOMETHING but not a lot. How that intersects with AW is kind of anyone's guess though.
The second quote in fact makes me doubt there's anything at all similar to AW there. It sounds more like an old-school survivalist game in the same vein as Aftermath or Twilight 2000.
You're right that the "how" benefits from learning what makes it work from the innovators themselves. Through study and play. But then the work is the plain craft of design. The challenge isn't innovation but integration and playtesting. Seeing what the purpose is of each element, and not stranding elements pointlessly. (I'm sure we can both think of examples of that!) Done well, it results in comments like this
But, again, this mostly evokes the old school. I'm getting an idea, yes, but it could have been concisely conveyed in terms of game design concepts by calling it a GNS S-type post-apocalypse genre survivalism focused game. Now, maybe there are additional elements, I'm not sure what 'indie RP elements' he's talking about, but sandboxed meta-plot, well, I'd have to see it to understand, that's for sure. I mean there's no way to know what all sorts of unique combinations of game elements MIGHT be possible, but overall given the BRP-like core design and very trad-sounding statements I'm in the dark and I'd guess overall we're probably looking at limited coherency '90s style 'trad with a currency' or something. Could be totally wrong of course...
It would be a d*** shame if designers weren't doing it. I absolutely want to see brilliant ideas jolting enduring modes into updated forms! You may have noticed me introducing the label "neosim" into some of my posts... that's in the direction of what I'm thinking of.
So do I, and IMHO, and my limited RPG game design experience, the way that I've found which points in a direction that is fruitful is to go look at the works of Baker/Edwards/Harper/et al and how they think about play, and design around that. It WORKS! I mean, my game may not be some brilliant work, I can't say personally, but it WORKS. It is coherently playable as it stands, and each part contributes. Where it falls down there are clearly diagnostic lessons to be drawn from the above experts which point towards improvements. GDS, 6 cultures of play, etc. never really did any of that for me. I can try to classify the design, and our resulting play, in those terms but it doesn't really lead to any conclusions about game design decisions or play techniques!