Obviously, that was not intended to be a full-functionality test, but it would detect any frequently-encountered display defect (or keyboard defect).

Calculator displays are multiplexed, so the usual defects are either one digit that never displays anything, or one segment that stays blank on all digits.

The defect mentioned by you is frequent only on displays with independent digits (like some digital clocks), not on calculators.

I do not know whether on calculator LCD displays there are frequent cases when a single segment can become defect.

At the time about which I am talking, calculators had either green vacuum fluorescent displays (like mine) or red LED displays. With such displays, the normal defects were either in the driving circuits or in the connections to the multiplexed display, so they affected either all segments of a digit or the same segment in all digits. I have never seen a case when the actual light-emitting segment of a digit of a VFD or LED display was defect.