That was only the proximate cause, the ultimate cause was cultural. As complex systems and efforts run into problems, it is trivial to blame the unit conversion when they had been ignoring people for months who had concerns [0]
> ... ground controllers ignored a string of indications that something was seriously wrong with the craft's trajectory, over a period of weeks if not months. But managers demanded that worriers and doubters "prove something was wrong," even though classic and fundamental principles of mission safety should have demanded that they themselves, in the presence of significant doubts, properly "prove all is right" with the flight
Dropping units on the NASA side also was problematic but really culture was the cause of the actual crash.
[0] https://spectrum.ieee.org/why-the-mars-probe-went-off-course
That's technically true, but if NASA's API accepted arguments in Meters instead of int32_t (or whatever the equivalent was in the language they used), then it would've been instantly obvious that the controller code that Lockheed Martin wrote was using the wrong datatype.
Do we know how the code was called, was it linked in or was it via IPC (the latter seems most likely to me, and then the question is does the IPC framework support a rich type system)?