I have gotten hold of the published book, the latest translation of the Medieval Text by Peter Jackson and read it.
I had an earlier discussion about Bulaks on Quora which resulted also on my Blog post at Bulaqs or no bulaqs in a medieval text. In short, as described by the translator the actual debate originated from the works of P. Pelliot, Notes sur l’histoire de la Horde d’Or who first propose the confusion between the Bulaq and the Vlachs given that the Mongols could not pronounce the letter 'B' and used 'u' for 'v'. However, Jackson accepts the suggestion made by Bratianu that Rubruck knew of the 'Bulaq' by their alternative name 'Lesgians' in the text. Currently, the accepted interpretation of the text is given by V. Ciociltan in his 'The Mongols and the Black Sea Trade in the Thirteenth and Fourteenth Centuries'. By Ulac, Rubruck meant to refer to the Vlachs living on the Northern shores of the Danube, in the region called Bugeac (or southern Bessarabia) about whom he knew from the '... Preaching Friars who travelled there before the Tartars came' mentioned at the end of that chapter XXI 'The River Iagac and various regions and peoples'.
After reading many more chapters of the book, I was under the impression that the Friar was only interested in discussing with the captured Catholic Christians of Hungarian and German ethnicity enslaved by Mongols during the Invasion of Hungary in 1241-1242, of whom he explicitly mentions a few words. However, in long Chapter XXIV in paragraph 56, the Friar mentions being involved in a debate between an 'Eastern' priest Jonas whose father was an archdeacon and thus know well the Christian Nicaean doctrine of Creation and an Manichean monk who claimed that
"Did not the Devil, on first day, bring earth from the four quarters of the world, make clay of it and them mould it into a human body, and God breathe a soul into it?"
I know that very few accounts on the actual belief system of the Bogumils exist. From the passage above, I believe we can get a glimpse of that.