No, it's not a conflict of interest. It's perhaps dumb, or morally bad, or several other things.
It’s sleazy because Lutnick’s son bet against the administration Lutnick was in, and against one of Trump’s signature policies. I’d be furious if I was Trump.
> dumb, or morally bad
This is easy to say in hindsight. There was a non-zero chance the decision could have went the other way. Also, companies aren't stupid. They don't buy insurance against things that are impossible.
And the supreme court doesn't hear cases that are 100% obviously illegal.