The claim he makes in this video is that "Hutu" just meant "peasants" and "Tutsi" meant "nobility," with the two groups being similar in culture, language, and religion. Superficial characteristics that German colonizers took as evidence of their being different races were just based on the Tutsi being taller and lighter, which, claims Feature History, were simply the marks of a privileged lifestyle rather than distinct ancestry.
I understand that ethnic and racial classifications are fluid, but would Hutu and Tutsi not have had any real conception of each other as "different" beyond wealth and lifestyle prior to European colonization? Or was it more akin to Norman rule in England, where ethnic and class distinctions were intertwined?
here's an answer from another thread that answers your question
https://www.reddit.com/r/AskHistorians/comments/1kkm29/how_can_the_hate_between_hutus_and_tutsis_be/
credits to /u/gplnd