but unfortunately I remember it even if that person was not important in any way in my life of course and I will never meet him again.

I have never taken the 5 year rule to  mean we can't remember things. I think it was more a comment about how we personally respond to things, as in overly reacting to small things. It was a nun speaking, remember. She went on to say that what was important, the things we should be concentrating on, was how we lived our life, our core values. She didn't say to forget it (just said the score itself would be irrelevant in our lives in 5 years), just to take a lesson from it, see what we could learn from the test.

Actually if we turn your incident around, what we could take from it is that rudeness counts. The man may have thought it didn't matter, you were a stranger,  who cares etc

But what it shows is that our everyday behaviour does impact on others more than maybe we realise. A little kindness goes a long way.

Your quotation reminded me of another one:

“Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid.”

(attributed to Albert Einstein)

The moving finger writes; and, having writ, moves on:

nor all thy piety nor wit shall lure it back to cancel half a line,

nor all thy tears wash out a word of it.

Omar Khayyam

“When all the trees have been cut down, When all the animals have been hunted, When all the waters are polluted, When all the air is unsafe to breathe, Only then will you discover you cannot eat money …”

― Cree Prophecy

“If you are depressed you are living in the past.
If you are anxious you are living in the future.
If you are at peace you are living in the present.”

― Lao Tzu

Actions speak louder than words, and are more to be regarded.