MATLAB Can't Manipulate 64-Bit Integers
And are afraid to tell anyone. Another possibility is they fired 'the guy' who knows how everything works and now they can only make cosmetic changes.
Tools For Understanding Code?
Try to change something. Maybe try to fix a bug, something repeatable, but non-cosmetic. Guess names and grep for objects that look like they have about the right name, put in a lot of 'log print' statements and run the thing, adding more log printing as needed. Repeat this every day of your life for about a year.