That’s only a modern day theory and feels somewhat retrofitted interpretation of the facts (from what I’ve seen when researching this myself). There are various spellings of the surname, since names often didn’t have a formal spelling in the early days of book keeping. And some of the spellings begun with a ‘T’.

So the more likely scenario is the name was only picked because it sounded similar to their original German family name while being an English-styled spelling.

Given we are taking 30+ years ago though, the best we can do at this stage is speculate.

However even if the card game theory were true, it still doesn’t make Trump a powerful name in the U.K. because the fart connotation is still more prevalent.