What I know for sure is that when Nostalrius first started, years ago, it was based on Mangos Zero project. I don't know about the current version, but it very very unlikely it was migrated to another core. Otherwise all previous work would have been lost.
What you heard is probably that Nostalrius core was added some Trinity core interface. It allows to use scripts that were originally written for trinity core. In that case, it is only the interface between core and scripts, but the core itself is not changed, it is still Mangos. I think it is quite a common practice on servers that have a dev team.
