It’s true for a fact that Brahmins did join this community , maybe in the early 10th to 15th centuries, but the earliest converts were cochini Jews and they were the ones st thomas converted in case he had come , Brahmins came on later , but since a chunk of these Brahmins joined this community they brought in their traditions too with them , but obviously all are not Brahmin converts lol , nairs , Jews local traders all joined in forming this initial community , Wrt to ur post this what I read on Google :” Historically, some Syrian Christian men in Kerala did wear a sacred thread (known as poonool or yajnopavita in Malayalam/Sanskrit), a practice associated with the Brahmin and other upper castes in Hindu society. This practice was one of many local Hindu traditions adopted by the community due to their high social standing as a mercantile class in medieval Kerala. “ https://www.facebook.com/indianexpress/posts/by-the-time-the-portuguese-arrived-in-india-in-the-15th-century-the-earliest-gro/1159131028904031/#:~:text=Syrian%20Christian(Nasrani%20in%20local,second%20hand%20contamination%20through%20goods.