As you suggest, the essence of a cult for most people is sequestration from "outsiders", and control through coercion using isolation and threats thereof.
The major religions have all gone through periods where you weren't supposed to engage those out of the faith. They usually back off (some would say for economic reasons) but some of the last phases are where one is not suppose to marry, much less have offspring outside of the "faith". There are still religions in this country today with much of this. Amish, Pennsylvania Dutch, Orthodox Jewish, and some versions of Islam still have rules along these lines to varying degrees. One of my employees has a daughter that was raised knowing that she must marry "within the Southern Baptist Faith" or the man must at least convert in order to have his "blessing". I note the son didn't seem to care much though. My sister married a Southern Baptist raised man, and his family refused to attend, other than his brother. A friend 50 years ago married a Hindu/Indian gentleman and the family refused to attend, visit, or otherwise engage this "western Christian woman.
The question becomes when does this pass from being a "cult" to being "tribal"? As with most things there is probably not a "line" but a "region". But in many ways for me, the dividing line is how one is treated if they leave the faith. Total separation and forcing others to separate too? Indifference? Or an acceptance that each must make their own choice.