You can also blame Chen Sanping, who I think lives in the West and associates with Victor Mair, an American professor of linguistics who has long tried to argue/insinuate Chinese civilization came from Indo-Europeans and that Indo-Europeans basically germinated all nomadic peoples (standard neo-Aryanism; Mair is of Germanic descent).
Politically speaking, the axis of "Japan-US-Chinese liberals (including DPP)" have historically despised Chinese nationalism, for obvious reasons - it weakens their ability to divide & conquer China. The Japanese came up with this theory during the 1930s clearly due to Japan's imperial designs on China and because they needed to challenge the traditional historiography of Chinese hegemony in East Asia. The Imperial Japanese were true Mongol-philes in that sense - they thought of themselves as the new Genghis Khan (warrior culture & such) and wanted to coopt the "Altaic legacy" in Asia even though they clearly were neither nomadic nor really related to the nomads.
Post-war, the Japanese stopped trying to go so hard on relating themselves to nomads, but I don't think they ever stopped trying to disassociate themselves with the "Chinese peasants." This is because admitting that China was the historical hegemonic power in East Asia would destroy their geopolitical aspirations, so they are incentivized to align with a narrative of "de-centering China" so that they could create an alternative power structure with them at the top (or at least near the top). Ultimately, the goal is to weaken and if possible Balkhanize China, as it is within such a geopolitical environment that Japan is able to most thrive.
Naturally this aligns with the West's aspirations, as well, and it also plays well into the liberal tendency to fragment & deconstruct everything. So it's a natural born match.
Which brings us to the Chinese expatriates like Chen Sanping. Why they support this, you can pretty much already guess - it's to ingratiate themselves within Western power structures, and also because they are true believers in liberalism, in a way that the Japanese and the Western mainstream are not. I mean just look at Trump and Takaichi. Do you think they're liberals? Why is liberalism losing steam in the West? Unfortunately Chinese liberals still haven't gotten the memo. It's like how Taiwanese may well become the last liberals in East Asia.