That’s likely apocryphal.

Morse requires you to know when the tone both starts and stops in order to differentiate a ‘dah’ from a break indicating a new letter or word or even just a ‘dit’.

Tapping a pencil only gives you the start of a tone since the pencil lift is silent. There is no real way to distinguish between a short tone and a pause (letter e) and a long tone (letter t) if you don’t know when the tone ends and a pause begins.

The same trope is shown in movies. You cannot tap Morse code if the recipient cannot hear when the tap ends.

In other words, the receiver has no way of knowing whether you sent an e or t without there being a signal that the pencil has lifted. Note that e or t can be substituted for any other number of paired letters.

It's very hard to decode morse from just "taps". You need to be able to hear two distinct symbols, "dash" (long) and "dot" (short) and the spaces. I remember in school we used to be able to do morse if we could see each other, using one finger for dot and one for dash, but we'd copy by looking at the fingers, not the sound of the taps.

Could have been a tap code https://en.wikipedia.org/wiki/Tap_code

Gotta give the kids credit for working so hard to learn a skill like that.