You're not wrong, and alternatives were considered, but those were really not fit to be ported to ISO Prolog in bounded time with complete lack of tests or even a basic reproducible demo, uncontrolled usage of non-ISO libs and features only available on the originally targetted Prolog implementation, and other issues typical of "academic codes."
The lack of unit tests is something I'm guilty of too and you're very right about it. The community is dimly aware of the fact that systems are more on the "academic prototype" side of things and less on the "enterprise software" but there's so little interest from industry that nobody is going to spend significant effort to change that. Kind of a catch 22 maybe.
How about ISO? Why was this a requirement, out of curiosity?