True. And even the English, who did both, struggled badly against the natives during King Phillip's war. Although I do think that Nordic warfare is more similar to NA warfare than that of Early Modern Europe. The pattern of raiding and counter-raiding would be intimately familiar to the icelanders, at least.A lot of those advantages are still true. But they didn't really fight on horseback and not sure we should expect the Vinlanders to be that good at dense formation fighting either. Iceland/Greenland didn't have military combat and military-type experience in new world will largely be for at least long time a mixture of low-intensity raiding conflicts with natives and ax duels with the guy who your wife had an affair with.
But iron metallurgy is still a huge advantage: there isn't much a guy with a flint or copper-tipped spear can do against mail, although I'm not sure how widespread that armor would have been.
AFAIK bison hunting only became a mode of subsistence once the Europeans created a huge market for bison products, which the Plainspeoples could exchange for firearms and horses. There's even a theory that that pattern of subsistence was not sustainable in the long term, and precipitated the collapse of bison populations in the latter half of the 19th century. Hence I doubt that even introducing the horse would replicate OTLs bison hunters. When we look at other horse-nomads in Eurasia and Africa, they are almost invariably pastoralists rather than hunter gatherers.There's a reason that farming peoples thrived on the Plains long after horses were introduced. The nomadic bison hunters relied on them for extra calories and goods.
Give them another couple of millennia and they might domesticate bison though.