Trade and commerce with Saudi Arabia is not unheard-of. It is believable that Trump registered companies in Saudi Arabia because he is businessman by nature and most likely saw some considerable opportunities there. It might not be clear whether what he did was right or wrong, and there are opposing opinions on the matter, but it is certainly likely that he did.
The Washington Post has found that Trump registered eight new companies during this year’s presidential campaign that “appear tied to a potential hotel project in Saudi Arabia.
Elsewhere, the Post has found that Trump has started companies that “range from sprawling, ultra-luxury real estate complexes to one-man holding companies and branding deals in Azerbaijan, Indonesia, Panama and other countries, including some where the United States maintains sensitive diplomatic ties.
Nobody knows what Trumps real plans and ideas are for the USA Citizens and the country as a whole. So Trump registered companies in Saudi Arabia which is business. Trump's enterprises have done business in 18 countries and territories across South America, the Middle East and Asia. The conflicts of interest are going to start piling up.
This is a story that keeps appearing in a variety of legitimate news sources, or at least outlets that require some vetting and verification before they publish. It's appeared in Vanity Fair, the Hill, the Washington Post, these aren't rags, and the stories are continuing to surface, so I think it's pretty likely that Trump did indeed register companies in Saudi Arabia.