Yes, that is true. If one enters IB as an undergraduate there’s not much upside to getting an MBA. The MBA route is taken after 3-5 years of relevant experience and one enters as an associate instead of an analyst. The analysts typically leave for a megafund or private equity after two years.
There are two main tracks to IB, undergraduate and MBA. Most of the non-target students enter through the MBA route.
This was a great thread on the subject moderated by @Catcherinthetoast : Want a Career in Banking? Ask a Senior Banking Exec