These people don't understand what ethnicity is
He's ethically Turkish. What he's not is someone whose ancestors came from Mongolia. But That's not what ethnicallyTurkish means. The Bulghars were a Turkic people who became ethically Slavic by assimilation, just like Anatolian Turks became Turkic via assimilation, with some slight Central Asian admixture
Turkic people are just one of the numerous steppe folk who have been traversing, and exiting, the Eurasian steppe for thousands of years, which has formed ethnicities from China to Western Europe. The earlier steppe folk came from the Western region of the steppe, and spread east south and west -forming European, Iranian Central Asian and South Asian/Indian ethnicities ) The later steppe folk, like Huns, Bulghars Hephthalites, Turks Mongols came from the Eastern steppe. The results of their migrations are no different than the earlier Western steppe migrations.