Both classified and top secret information.
For example, changes to China's production, mobilization, expenditure (sale of stocks, debts, etc...), changes to net imports/decreases in net exports, satellite imagery, State Dept/CIA field officers analysis/reports, etc...
I'd still say that by the end of january, there was absolutely plenty of reason to sell off stock. A reasonable person absolutely would have made that decision based on information that was already public.
The stock was sold immediately after a coronavirus briefing, not just some stock, his entire net worth.
Maybe it is reasonable to have sold your entire net worth based solely on the available public news, but then why not well before or well after the briefing and instead right after the briefing? Why tip of your wealth constituents at a $10,000/plate fundraiser while simultaneously maintaining a different political position publicly?