Because they made over $3 million probably having done little to no trading prior to that run.
Pretty much anyone could spot insider trading patterns. I’d be willing to bet these guys were making bets only on Netflix in the form of short-dated options most of which typically expire worthless. If someone were to repeatedly aggressively buy low probability short-dated options around an event like earnings and win consistently you would be able to infer that they are trading on insider knowledge.
Of course recognizing the pattern is only half the battle. They also have to figure out how they are accessing the inside info, hence tracing the traders back to Netflix employees and conversations they had regarding paybacks, etc.