A thong basically just means a thin strip of something, so you can see how it could refer to either flip-flops or scanty undergarments.
I admit to being unreasonably irritated if Americanisms are used in English. Smart is being used increasingly to mean clever. I keep seeing Mom for Mum. Bathroom, which has been used to mean lavatory in America for a long time, is being used more and more here. Silly euphemism - though of course lavatory is a euphemism too, but lost in the mists of time. In What Katy Did At School, published in America in, I think, the 1870s, lavatory is used to describe the washroom in a boarding school. This puzzled me when I was a child.
Other irrational irritations for me are the use of math instead of maths (fortunately not used in the U.K. yet), and the fact that Americans call the language they speak English. It’s American English, as opposed to proper British English. After all, we came first ?