Almost single major oil company & refinery has a team of traders that directly participate (or use the investment banks / futures brokers) in the crude oil and refined product futures / options markets. This is both for hedging and speculation. If a refinery knows when it's going down for (un)planned, it lets it's traders know and they do trade on that info. In commodities markets, this is legal and does not constitute insider trading.

if that were true they would just publicize their production numbers

They still don't want their competitors to know how well they're doing or their suppliers, who might increase their prices if they know the company is doing well.