This theory doesn't make sense to me, weren't there some some half Hunnic people in roman history? If they were half Xiongu, wouldn't they stick out visually so much that someone would mention this in the histories?
Or do I have some faulty assumptions? Turkish people don't look as different from Europeans as Mongols do
The reason that some historians believe that the Huns and the Xiongnu were closely connected (or, as you say, that the Huns were the Xiongnu) relates to both geography and time.
Both originate from the eastern steppe (roughly, modern Mongolia). The Xiongnu are a more ancient nomadic group from roughly the 3rd century BCE-beginning of the Common Era. The Huns came later and were at their height between the 4th and 6th century CE. The Huns also expanded farther than the Xiongnu, moving all the way from the eastern steppe into Europe. As you mention, there was contact between the Huns and the Roman Empire, and the Huns caused a refugee crisis of sorts on the fringes of the Empire as they pushed Germanic groups seeking refuge into the Empire's borders.
Thus, given the shared location of origin and the fact that the Xiongnu predated the Huns, historians have questioned whether the Huns were descendants of the Xiongnu, or if they were distinct nomadic groups.
Part of the difficultly of this question is determining what is meant by descendance: is it culture, or ethnicity, or a mix of both?
Some scholars have argued that there is enough cultural similarity (particularly linguistic and political) to connect the two, and some of the historiography (the history of the study of history) relevant to that point is discussed in this very brief book review of The Huns, Rome and the Birth of Europe by Hyun Jim Kim (Cambridge: Cambridge University Press, 2013) by Richard Payne.
When considering ethnic connections, however, the answer is far less clear and there is also far less consensus among scholars. Part of the difficulty in determining ethnic connections between the two groups based on bioarcheological evidence is the fact that both were nomadic. Both groups, in other words, were considerably genetically diverse (and the Huns especially so, with genetic markers connecting them to peoples from across Eurasia), making it difficult to determine direct connections between the two. There is a very recent thesis that outlines many of the salient points within this debate and argues that there is not enough evidence to confidently connect the two.
I hope this answers your question - I'm not sure I understand your point about the "Turkic" vs "Mongol" appearance. Remember that the Mongol Empire doesn't emerge until the later Middle Ages.
This has been asked before and given a detailed answerby u/flavivsAetivs