Shijing said:A: Keep in mind that he is not a "green horn".
Since "green horn" was given in quotes here, I wonder if that was an oblique reference to Jill Stein and the Green Party. If so, it's pretty on point. I respect Stein a lot, and she was my preferred candidate on principle given what I knew before the election, but if she were ever to be elected (very unlikely), my greatest concern would be that she'd be eaten alive by the establishment as soon as she got into office due to her relative lack of experience in the political arena. Trump technically lacks that as well, but as has been pointed out above his business experience compensates for this.
I think the reference is more likely to refer to 'an inexperienced person; a novice, beginner or newcomer' (Wiktionary).
Trump has masses of experience in negotiation and big deal making having built up his business empire. These assets may be invaluable to him as President and help compensate for his lack of political experience as you mentioned. In a similar way Putin had the benefit of his vast intelligence experience to draw upon when entering office.
There is also this from Wiktionary, which may reflect Trump's pragmatic approach and that he is less likely to rush into a course of action without due consideration. Like calmly focusing on his agenda amid the heat of the Washington political crucible, and therefore not reacting emotionally in the face of an unexpected event perhaps.
From 17th century jewelery manufacturing which used decorated horn which was impressed at a specific temperature. If heated too high (a common mistake by apprentices), the horn turned green.