slicing a cucumber is a common feature of youtube videos demonstrating how sharp knives are. (I spent a completely unreasonable amount of time watching knife sharpening videos on youtube a couple months ago)
From my own experience slicing slightly overripe tomatoes that have some give to them are much harder to slice with a non serrated knife than a firm cucumber. Wonder why cucumbers became the standard haha.
I've seen them also use daikon radishes to demo "sharpness". Maybe it is a true test, but part of me thinks this is like demoing how good wipers are by using window cleaner instead of water to demo how good they are.