Unfortunately, that's a limitation of the Twitter API, not of rtweet. They now have a tiered Search API, and searching a term from a specific user is one of the "premium operators".
They did a blog post when they rolled out the new tiered format, but I'm not sure how current it is:
It looks like the pricing starts at $149 per month.
For the specific case of Trump, there is actually an archive of all of his tweets, which you can export:
http://www.trumptwitterarchive.com/archive