Overthinked and overengineered describes Monzo very well. Their business model isn't really banking but coming up with ways to extract money from investors, so this kind of overengineering is a strong business move for them.
I have my gripes with Monzo’s business model and over-engineering (though that applies to most VC-backed startups nowadays) but security is quite important and this seems like a sane strategy of managing root certificates.