Best guess...Cuban doesn't like, or agree with, all the things Harris brings with her...policy, political beliefs, personality, etc...
But he feels she's a better choice than Trump is. So he's focusing on the things he thinks she's good at, or has done well, and is putting his face and name on it to try to help her beat Trump.
There are millions that have claimed to "not like Trump the person", but feel he's a better choice than the alternative. Many of you on this board make that argument, nearly daily. And in many situations, your support for Trump in the back and forths, ends up making it sure sound like you actually do like Trump the person, and all the "stuff" he brings to the table. I know tons of people that I fully believe don't like Trump the person, but support him because of their overall policy beliefs AND policy disbeliefs on the other side.
I assume this is what's happening with Cuban, right now.