Not sure what exactly the author does, but IM is the absolute life saver in every developers world. E-Mails are spammy, include too many people, are too long, take too long to write, get drowned in a sea of useless crap, etc... Everyone working in a bigger corporation, knows that emails are a major time sink.
Video calls? Sure, if there is a meeting. But video calls, not matter what software you use, is a nightmare. The quality is crap, the connection lags and it never feels anything close to real life interaction...
But most importantly, IM let's you respond on your own time. There is nothing worse that people talking to you or calling you with a video call while you are trying to figure out why that statemachine recursed into the 20th method call and then jumping this way and that way, suddenly jumped the wrong way and then still produced the correct result while thinking about how it got into the 20th call in the first place when it should have branche din the 10th call... You get the picture.
And whops. There we go. Good old Tim stopped wanking to Day-Z and got some shit done, all on his own.
Not to mention that Epic Games is pile of crap now that they released this game. It was once a nice company to work work for, now its just the same shitty sweat shop like any other gaming company, who only sees bottom lines and milking customers... And employees.