This happens all the time, to the point we have a name for it (Not Invented Here syndrome).

That it took so long before they started trying to phase out their home-rolled library for the "hard cases" is somewhat unrealistic, although possible in a sufficiently-dysfunctional organisation. Some of the details about the problems of their homespun library are clearly anecdotes translated from other settings, and are unrealistic in the context of a mathematics / finance library. (They only noticed that interest calculations were wrong when a customer complained? Seriously?) The development of 6.1.0 (via 6.0.0) taking only two weeks isn't congruent with the rest of the story, although it may be realistic for AI-driven development (with which I am unaccustomed).

But otherwise, this is one of the more realistic satire pieces I've read.