There doesn't appear to have been any significant population decline in eastern North America around the time the Norse arrive (circa 1000 CE). In fact, this was a time of considerable population growth as maize-based agriculture was rapidly spreading east at this time. The Norse themselves didn't have the same disease load as mainland Europeans further south (Iceland didn't get measles until the 1200s, for example, well after the Vinland outposts in North America were abandoned).

After Columbus, however, there was more direct connect between the Americas and mainland Europe, which allowed for more rapid exchange of diseases. There were many different epidemic diseases (not all of them from Europe) that tore through the Americas at this time. Diseases weren't the only factor at play here - and war, slave raiding, forced labor camps, droughts, etc. all contributed to compromising the immune systems of large populations which in turn allowed the epidemics to spread. The 90% figure comes specifically from long-term colonial census data from Mexico and reflects deaths from all causes. Whether all populations suffered similarly is debatable - some places weren't hit as badly, others were actually hit worse. /u/anthropology_nerd has a great post about this here.

allowing the English and other nations to dominate when landing later.

In New England, there's definitely some truth to this. When Europeans first arrived in southern New England in the 1500s it was was rather densely populated - so much so that the French thought it best to turn their attention further north. An unidentified epidemic struck the eastern North America, from New England to Virginia, between 1616-1618. The people of eastern Massachusetts were hardest hit, with some villages being completely wiped out. However, fewer people died the further west the disease went (the Pequot in western Connecticut were barely affected and further afield in Virginia, reports of deaths from this epidemic are few and far between). Because of this disparity, the Wampanoag Confederacy in eastern Massachusetts allied with the newly arrived English colonists in 1620 because they were concerned that their western neighbors would take advantage of their current weakened state. This gave the Plymouth colonists a foothold in New England. In the 1630s, the first smallpox epidemic struck the area, doing further damage to local populations. The pious colonists viewed the various diseases sweeping over the land as a sign of that the continent was their divine inheritance.

The top comment doesn't read to me like he is saying the Vikings brought smallpox. He states it came afterward, with Columbus.