"While it is true that the United States Constitution does not permit any citizen to hold a title of nobility “from any King, Prince, or Foreign State” under Article 1, Section 9, Clause 8, that doesn’t prevent anyone . . . from holding an “honorary” title. Indeed, honorary knighthood is not full knighthood but an award presented on behalf of the Sovereign to any non-Commonwealth citizen whose work has benefitted the United Kingdom.
"Several Presidents have been knighted by the Queen, including George H.W. Bush and Ronald Reagan, but Eisenhower has received honors from Her Majesty twice (technically three times). In 1947, he was honored as a WWII military leader as a Knight Grand Cross of the Most Honourable Order of Bath and the Order of Merit. As President of the United States, he was honored again with the GCB in 1957."
https://anglotopia.net/british-history/ten-americans-with-honorary-british-knighthoods/