I'm a papaya simp, but Oscar and Lando were running consecutively when all this kicked off and I agree with the stewards on this. Let's look at the data.

Gap when sector 2 caution was flown: 2.9s

Gap when safety car was called: 2.8s

Gap when arriving at pits: 7.3s

At the beginning of that lap, Lando was instructed on the radio to give 100% pace. They were in the lap window for the first of a two stop strategy, and that direction usually means that pitting is imminent. A safety car in that timeframe is a literal no-brainer for a stop.

Lando asserts that the team did not direct him to box until he was most of the way down Casino. That's technically true. However, when Lando was at the exit of turn ten, he received the radio message "Lando, safety car, safety car, you are the second car, you are the second car. Oscar 3."

Lando already knew that Oscar was the car ahead. He had overtaken Nico at the tail end of the previous lap, and even if he forgot that his teammate was the preceding car to do that, they were accordioned in 10. Once both cars were on Casino, Oscar accelerated off at a pace sufficient to cause the car to scrape down the track. Lando was on-off throttle and was weaving around.

This can only leave us with the obvious, which is that Lando received and acted upon tacit instruction to build a gap between him and Oscar to allow for the double stack.