Trump Media is now a company. But is it a business?

Susquehanna International Group makes its money buying and selling stocks in milliseconds, eking out fractions of a penny each time. At one millisecond at the end of last year, its holdings included a 2% stake in a blank-check company that this week merged with Donald Trump’s social-media company.

That has fed theories that Susquehanna’s founder, Republican mega-donor Jeff Yass, is funneling money to a cash-strapped Trump by pumping up shares of a company that began trading today under the ticker DJT and owns Truth Social, where Trump has spent most...