I do not think this is a mistake. Remember how Cruz got booed for his non-endorsement. No harm in doing this.
No harm in endorsing a destroyer?
I thought he had already endorsed Nehlen.
I think the best thing ANYONE can say in primaries is
“I wish the best of luck to all the primary candidates and I will endorse the winner.”
(Maybe)
The harm to Trump is how this affects how his supporters view him. He's endorsing an open borders advocate.