Thier were some easily spotted problems that needed fixing. In this case they applied a very simplistic version of the "windowing" solution. The actual recommended algorithm was to check if the twodigit year was less than 20 years from the current two digit year. A bodge which would have worked up till 2079.
I think the major problem here is that the people writing these systems in the 1970s and 80s never imagined they woul still be running 20 plus years later, and the poor sods patching up the systems in the late 90s never imagined a company would be stupid enough to be running such antiquated systems twenty years later.
Here I should confess to my own personal Y2K bug. A routine I wrote circa 1988 would follow the 28th February 2000 by another 28th February 2000 .. oops.