Earlier quoted context omitted.
Perhaps... but I think companies like FB also get (total guess) thousands of resumes per day. So, for every engineer that walks away, many more are knocking at the door. So, I think in management's eyes, very very very few people are irreplaceable.
That's not true at all where I am. Yes, I can post on twitter and get tons of CVs, but most applicants aren't that good. I may only want to interview 40% of the candidates I get, and I would really only want to hire about 1 in 5 people I interview. Interviewing takes time. Then after that, onboarding people on the job takes time too, it can take people several months to become effective on the job. Skilled, effective…
Look at any failed multi-decade war waged by any unbeatable superpower past or present to see the highest levels of military and civilian command employ these exact same methods to build careers so successful that they'll be immortalized in statues for centuries after their deaths :)