Disclaimer: All of the below is pure hearsay. It is not based on hard evidence, but on testimonials of individual people. People can lie and make stuff up. People can troll just for the fun of it.
That being said, I am quite convinced that it is at the very least partially, if not almost completely, true.
There are a few big game companies who are absolutely notorious and infamous about how tyrannical and oppressive they are towards their employees. Perhaps the most notorious one is Electronic Arts. However, they are by far not the only one. (Ubisoft has also gained notoriety on this front in later years, as another example.)
Have you ever thought how some of these companies can publish a dozen or so triple-A games with humongous production values every year? How they can more or less meet deadlines, even though projects of this size are notorious for always going way over timelines and budgets? One way they do is to push their employees to the absolute limit. Up to a point that they could pretty much be called slave labor.
60 or even 70 hour work weeks are common practice. Even 80 hours per week is not unheard of. And since we are talking mostly about the United States here, everything that's over the standard 40 hours is completely and absolutely unpaid. The employees are essentially pressured into working for the company for free (with the threat of firing them if they don't complain.)
And even those paid 40 hours aren't necessarily that lucrative. In many cases these employees are actually being paid less than mandatory minimum wage, when you take into account all the hours they do.
They might, might, get a bonus once the project is completed, but said bonus will seldom be even near worth all those extra unpaid hours. Only a small fraction of it.
At some places there has even formed a group psychology between the employees themselves. Anybody who leaves the workplace too soon, before the others, will be scoffed at, and considered a traitor. Many of these employees do so many hours every day that even a factory worker of the late 1800's would have thought it too much.
And do you know what happens to many of the employees once the project is completed? They get fired. Just like that. (Again, since we are talking mostly about the United States here, employees have essentially zero governmental protection, and employers can do pretty much whatever they like.) I suppose that, somehow, these companies think that it's cheaper to employ new college freshmen to replace the older employees, than keep the old ones.
This is pretty much slave labor. The amount of unpaid work hours is just amazing. Sometimes even as much as 50% of all the work that these people do is completely unpaid for. And they are effectively forced to do it (or face getting fired).
Of course not all game companies are like that. Some are (or at least claim to be) very fair and pay for every minute of work, and have reasonable working hours. But these seem to be more the exception than the rule, at least when we are talking about the major game companies who produce tons of triple-A games.
It is no wonder that many indie developers are ex-employees of these huge companies. Either they got fed up and left, or were fired (often for no reason at all, other than to save money.)