This interpreter was obviously not written by hand, the author has likely written a compiler from a sane language to malbolge and used it to obtain this result. It would be much more interesting to see the source for this compiler to malbolge or have an explanation of how it works.
A C compiler that can output Malbolge (among many other esolangs) can be found at https://github.com/shinh/elvm - dunno if it has any relation to this work.
Note that the Malbolge (or rather LMAO/HeLL) backend was added back in July 2019, so I'm not so sure about their claim that "It's as of 2020 and 2021, the most advanced, usable Malbolge program ever created."