Maybe Trump thought it sounded less harsh and combative than enemy
Trump doesn’t think. And this is a case in point. Transcript of the interview:
JEFF GLOR: Who is your biggest competitor? Your biggest foe globally right now?
PRESIDENT TRUMP: Well I think we have a lot of foes. I think the European Union is a foe, what they do to us in trade. Now you wouldn’t think of the European Union but they’re a foe. Russia is foe in certain respects. China is a foe economically, certainly they are a foe. But that doesn’t mean they are bad. It doesn’t mean anything. It means that they are competitive.
Note, he didn’t choose the word. He was prompted by the CBS interviewer. (And in doing so, he failed Media Training 101: Never repeat a bad or leading question. Reframe it and answer in way that you want.)
In this particular case, I think the subsequent commentary has genuinely (and perhaps deliberately) misinterpreted what he said. The interviewer equated foe with competitor, and that’s how he answered it. Nor was the interviewer particularly off base (another baseball metaphor) in using the word. From the OED:
B.1.c. In weakened sense: an antagonist, an opponent.
1697 Dryden tr. Virgil Georgics ii, in tr. Virgil Wks. 94 The Groom..watches with a trip his Foe to foil.
1795 Hints to Opposition: Let. to C. J. Fox 2 I am not of consequence enough to have either political friends, or political foes.
1870 W. Morris Earthly Paradise I. 133 Made happy that the foe the prize hath won.
1949 Charleroi (Pa.) Mail 23 Feb. 7/1 Lee nearly nailed his foe in the first round with a flurry of lefts.
1979 Whig-Standard (Kingston, Ont.) 2 Apr. 4/2 John Diefenbaker [sc. a politician] is 83 years old, but still a formidable foe.
2000 Sunday Mail (Brisbane) 5 Mar. 21/7 Old political foes Sir Jon Bjelke-Petersen and Peter Beattie have joined forces again.
Note the later cites are all from North America/Australia.
A more skilled politician wouldn’t have repeated the word, but Trump wasn’t calling the EU an enemy. At least not in this interview.
(I hate it when I have to defend Trump.)