Travel broadens the mind. Allegedly.
You cannot visit a town in Italy which does not have a tourist shop - at least one - selling Leonardo da Vinci paraphernalia. Or has a Leonardo "museum".
And thus here we have a typical tourist trap shop with the Assisi T-shirts (fair enough), and the Italia T-shirt with the clichéd symbols of a gondola for Venice, the Colosseum for Rome, the leaning Tower of Pisa, and so on (also fair enough, I suppose) and... T-shirts for Leonardo da Vinci.
Who in the what now?
There is no indication that Leonardo ever visited Assisi. I cannot find any of his paintings on the subject of St Francis. Who, incidentally, died (in Umbria) 226 years before Leonardo was born (in Firenze, Toscana (Tuscany) in 1452), so they were hardly contemporaries. I am not aware of any of his works that relate to Assisi. He worked mostly in Firenze, Milano, Roma, and France. It's not impossible that he visited Umbria at some point; after all it neighbours the provinces that he was living and working in for a significant part of his life, and it's a very pretty place to visit.
But equally, there were no Frecciarossa trains running in the 1400s. To visit Assisi would have involved a very lengthy, tiring trip.
But just like the song "That's Amore", T-shirts featuring the "Leaning Tower of Pizzas" (sic), and baseball caps with "Italia 1861" (when the process of unification wasn't really complete until 1870 since we were still missing a couple of important pieces like, oh, I don't know, Roma and Venezia in 1861), how the hell would the tourists know any different?
I suppose it's no different to tourist shops in George Street Sydney selling road signs that say "Kangaroos, Next 25 km". Or worse, "Crocodiles". In Tasmania.
I implore you to select your mementos wisely. This is not it.
(I'm not going to name the store in question even if I could find it. People have to make a living, and I suppose they're just giving the people what they want. It would just be nice if the people would decide to make more informed choices about what those things are.)