“But true scallions are actually a distinct species.”
Ah yes, the “no true scalli–” eh, Scotsman fallacy. 
I’m pretty skeptical about this. Wikipedia says " Scallions (also known as green onions or spring onions or sibies) are vegetables derived from various species in the genus Allium." I didn’t check to see if they have a robust reference for that yet – can confirm the NYT had none! 
I’m not sure if we have exactly the same range of species and varieties here as in the US, but that whole group would be happily sold as “spring onions” in the UK – though hon. mention to the Scots word syboes/sybies! – and the very same ones would all “scallions” in Ireland. More power to you in making finer-grained distinctions, but good luck being 100% consistent if you’re going by vagarities like precise bulb convexity!
Allium varieties are fairly tricky, mind you, and there’s also the issue of cultivation and packaging. Often scallions/spring onions would grow into something rather more oniony if left to more fully mature (whether or not it’s any of the commercially used varieties). And infamously of course, some “garlic” bulbs are actually leeks!
