the forebrain, and hindbrain, dont merge during development, and they are not derived of radically different cells, and they originate from the same process. the difference between them is expressive regulation.

wikipedia being what it is, on par with secondary literature gives an overview of brain and nervous system origin.

https://en.wikipedia.org/wiki/Development_of_the_nervous_sys...

the primary discipline is embryology, and if you view the progression from cellular to somatic, it is quite clear that structure of nervous system, and brain are early events during organogenesis, and are derived from contact and signalling between cellular plates. there is a gradient derived signal, that induces disparate potency of genetic expression. there is however, not , two individual centers of cellular genesis, all neural cells are derived from same cell cell interaction, and induced to take a constrained set of functionalities based on signal amplitudes.

https://embryology.oit.duke.edu/neuro/neuro.html

the original research in question did not discover a two brain merger, thats a tragic misinterpretation that is doing disservice, and is not even the claim of the research.

the discovery is the difference in genetic expression that required a different cultural regime for hindbrain neurons invitro. previously the paradigm was that forebrain, and hindbrain required almost similar culture, and this discovery allows realization of that fact, leading to a cultural regime promoting invitro hindbrain neuroculture.