No, but if he loses, no more donations will come in, and since the loan is specifically secured by his securities, in effect, he is betting his assets.
This is just a smarter way to do it than selling them up front, because the capital gains taxes are a lot higher than the interest on a secured margin loan.
The article states the Cruz family was bringing in more than a million a year. I think perhaps you are letting your imagination build up Cruz's great big "sacrifice" here.