“This gift is a major reason he did very well arguing before SCOTUS.”
Attorneys don’t “argue” before SCOTUS. They have precisely 30 minutes to READ their documentation supporting their argument. The documentation is the result of weeks or months or even years of research done by the attorney’s colleagues, paralegals, etc. It doesn’t require “thinking on your feet”.
It’s easy to see why he bombs in rallies and debates.
With the exception of Justice Thomas, SCOTUS Justices ask questions that are frequently designed to expose holes in the legal arguments made by the attorneys.
The attorneys must successfully argue by filling those holes with satisfactory answers. This includes citing case precedents, statutes, and regulations. Cruz was able to do that very well as shown by his track record before the court.