Well, it's not happening now. It's been happening gradually for a long time. All my friends in tech including FAANG have been working remotely to a certain degree. Some only once a week, others twice or even fully remote. And even those in the office work mostly in "remote" mode since they need to collaborate with people in offices all over.

For big companies it's harder to work fully remote. There are too many people and teams and dependencies. Twitter is not small but also not big.