Does this recipe really only require 100g total flour?

Mariss -
You were right to ask :)
The recipe as first posted had omitted 275g bread flour, which has now been restored.
See also this similar version of the dish, from another website, which uses 350g bread flour
http://bettysliu.com/2016/12/16/black-sesame-tangzhong-milk-bread-babka/

Where are you finding one cup of flour? I see 25g of flour in the tangzhong slurry and another 350g of flours in the final dough. That's closer to 3 cups than one cup. It looks tasty!