I stumbled on this Feb 15 2020 picture taken at Sushi Yasuda in NYC. I'm flabbergasted that you can make nigiri with chives or scallions that I would've never thought of!
— This part refers to a discussion about the sushi being topped with chives. —
If these sushi were topped with chives, that would be very thin chives indeed (or possibly new growth, but for that, they seem too dark).
This is one average-sized chive next to a few grains of cooked long grain rice. You’ll notice that the diameter is roughly the same, unlike in the photo in the question.
— end —
So let’s assume that the caption of the photo is correct and we are indeed looking at scallions. In that case, it would be first-year scallions just grown from seed which are usually not harvested for sale. But as barely-more-than-sprouts, I would expect them to be way milder than mature scallions or mature chives - and that’s what various sellers claim as well. But they would still be “regular” scallions or onions, not a special breed. The suitability for the intended use is based on the very young age.
As scallions are usually marketed as mature crops, you may classify them as „special produce“ and I would guess that you won’t find them for sale except in specialized stores - as micro greens, for example:
(screenshoot from previous link).
Starting your own crop from seed may be necessary to recreate this kind of sushi. I would hesitate to use chives or mature scallions, as both would be very pungent. And after one “oniony” sushi, your palate would be spoiled for other kinds. Whether it’s really as simple as putting the scallions on top of the rice is unclear to say from just a photo - there’s certainly room for your own creativity.
To expand a bit on LSchoon's last point: Biting into a bundle of chives like that would be disgusting. While less intense than other aliums, chives are still fairly piquant, and the amount on one of those pieces is more than I'd use in an entire meal for several people. One bite of a bundle that size and you'd be tasting chives -- in both directions -- for the rest of the evening.
Now, assuming the sushi chef was neither incompetent nor actively malevolent, it's likely that those were special chives in some way. Very young, freshly picked chives might have a more delicate flavor, and the chives might be a specific cultivar which is less intense in general. But if you just purchase a bag of chives at the store, I would not expect the results to be pleasant.
