Has "selfish mining" ever been observed among miners?

I recently watched a video from Emin Gün Sirer, and he spoke a bit about his work on selfish mining. The idea being that instead of broadcasting a block, a miner could benefit from keeping it secret and working on the next block. He said that in simulations, it turns out to be a superior strategy. (although my understanding is it requires a significant percentage of the overall hashpower)

If it is a superior strategy, then we should have seen it happen in practice. Has it? I remember reading debates about this back in the day, but didn't follow it closely.