How rare is this level of ticklishness?

Have two female friends who, when tickled on feet and ribs, but most especially on feet, they cannot even form a single word (in any languages they know, mother tongue or otherwise), and all they can utter from their mouth when having their feet tickled are some totally nonsensical non-words. (For example, say when they want to say "I am too ticklish', they may utter something like "Omes trou lageur fahr".)

They have played games with friends in which if they can form any sensical sentence or even phrase (such as "today is Monday") in a session, they have a silver-win, and if they can form some specified sentence or even phrase (such as "begging to stop"), they get a gold-win. Normally the session lasts for 15, 20, or 30 minutes and stops as soon as they form any sensical sentence or phrase, but as far as I know, they have never won a single session.

So how rare do you think is this level of ticklishness?

Nice fantasy about the two friends