A mysterious cloud was indeed recorded in 536 by contemporary historians, but its effects are definitely exaggerated in the article. This is made worse by the /r/todayilearned title:

TIL In 526 there was a worldwide dust cloud that blocked out the sun for an entire year. It resulted in widespread famine and disease. More than 80% of Scandinavia and China starved to death, 30% of Europe died and empires fell. The cause is unknown.

This is just a bad summary of a bad article. For a start, they got the year wrong! Famine and disease were alluded to in the sources, such as by John Lydus, but no numbers were ever given, which is notable when compared to the plague of 541, which was described in far more detail by sources such as Procopius and Evagrius. Moreover, we can't really take sources at face value like that. John of Ephesus for example was a bishop writing in a highly religious context and arguably exaggerated the evils of the world in order to urge others to embrace a holier life/abandon the corrupt world they lived in, so of course he wrote about disasters in a very hyperbolic fashion. Yet for this mysterious cloud, all John said was:

The year 842: the sun darkened and stayed covered with darkness a year and a half, that is eighteen months. Although rays were visible around it for two or three hours (a day) they were as if diseased, with the result that fruits did not reach full ripeness. All the wine had the taste of reject grapes. Then the Lord let his mercy appear to his creation and shine upon it. And again the people reverted to hardness of heart without fear, just as before.

Compare that with this brief extract from his much longer description of the plague:

Thus when I, a wretch, wanted to include these matters in a record of history, my thoughts were seized many times by stupor, and for many reasons I planned to omit it, firstly because all mouths and tongues are insufficient to relate it, and moreover because even if there could be found such that would record (at least) a little from among the multitude (of matters), what use would it be, when the entire world was tottering and reaching its dissolution and the length of generations was cut short? And for whom would he who wrote be writing?

I think it is quite clear that for John the plague was the earth-shattering event of recent history, rather than the cloud, which was a sign of God's dissatisfaction with mankind, but not much more than that. The same can be seen in the Chronicle of Pseudo-Zachariah of Mitylene, who also gave the cloud a short and pithy description:

The sun began to become dark at daytime, and the moon by night, while the ocean was stormy with spray from the 24th of the same month of this year until the 24th of June of the following [indiction] year fifteen.

Likewise for Procopius:

And it came about during this year that a most dread portent took place. For the sun gave forth its light without brightness, like the moon, during this whole year, and it seemed exceedingly like the sun in eclipse, for the beams it shed were not clear nor such as it is accustomed to shed. And from the time when this thing happened men were free neither from war nor pestilence nor any other thing leading to death. And it was the time when Justinian was in the tenth year of his reign [536/37].

The cloud was a dark portent and Procopius hinted at a grim future, but the cloud was clearly not the sole cause of human suffering, especially as Procopius later described how many people died because of the plague/Justinian's wars.

As an aside, I would like to point out that Michael the Syrian's account of the cloud cited in the article was not 'contemporary' - he lived in the twelfth century! His evidence is valuable because John of Ephesus' Ecclesiastical History covering the early sixth century is lost, so the fact that Michael (and other later historians) epitomised John's history is the only reason we still have 2/3 of his contemporary ecclesiastical history. John's work was however also used by Pseudo-Dionysius of Tel-Mahre, who actually said outright that he copied John's work verbatim. Michael the Syrian and a few others basically summarised Pseudo-Dionysius' history for events in the sixth century, so there really is is no reason for the article to appeal to both authorities, as we have the fuller account from Pseudo-Dionysius.

Anyway, back to the strange cloud. I've heard about this before, but have never really connected it to economic decline in the Eastern Roman Empire. The conventional explanation for that is the plague, which had a high death toll across Europe, though we can't be sure of how big a demographic decline it caused (people claiming a specific figure are just quoting contemporary writers, who do not provide reliable numbers!). The cloud certainly had an impact on agriculture, as several of the journal articles cited agreed, but it would be a mistake to blame an unknown number of deaths on a mysterious cloud that we don't even know the cause of. Archaeological evidence for instance does suggest that there were significantly fewer occupied sites in Scandinavia in the sixth century, but at no point did the journal articles referenced ever suggest that up to 80% of the population died or that it could only be due to the cloud, especially as the plague followed only a few years afterwards. The same must be true for China as well.

So yeah, the cloud did appear in 536 (can't tell you why I'm afraid, not a scientist!), but I doubt that it was as impactful as the article suggested. I'll end by quoting Antti Arjava, the historian who wrote most recently on this:

The literary sources that record the darkness of 536/37 all seem to consider it a temporary misfortune. Among the innumerable earthquakes, droughts, plagues, swarms of locusts, and slaughters that are listed by the historians of this time, the dark cloud was not considered a particularly severe catastrophe. Shortage of food was recurrent in the ancient world, and people were used to it, however intense the short-term suffering might be. [...] Thus the combined force of the available evidence irresistibly shows that, whatever happened around 536, its historical implications remained limited, at least in the Mediterranean area.

Sources:

The various primary sources I used are named. As for secondary stuff, the article actually cited many credible sources, but it did not summarise their conclusions accurately. Any one of them would be a good introduction to this topic if you can access them. The three below are the three articles that were the most useful/interesting for me.

Arjava, A., The Mystery Cloud of 536 CE in the Mediterranean Sources, Dumbarton Oaks Papers (2005)

Graslund, B. & Price, N., Twilight of the gods? The ‘dust veil event’ of AD 536 in critical perspective, Antiquity (2012)

Löwenborg, D., An Iron Age Shock Doctrine – Did the AD 536-7 event trigger large-scale social changes in the Mälaren valley area?, Journal of Archaeology and Ancient History (2012)