Might be true for some people, but it's not the primary motivation for freelancers. Usually it's the highly-skilled, highly-paid people who are in a position to pick up freelance work, not your lowest-paid workers at the bottom of the hierarchy. They do it not because they need the money, but because it's there for the taking and they get to work on something interesting while they do it.

With a salaried dayjob, your effective hourly rate goes down with each additional hour you work. With a freelance job, your effective income goes up with each additional hour worked.

Someone who has both types of job at the same time has an incentive to minimize their input to their dayjob so they can maximize the incremental income from their freelancing work. Sure, theoretically a perfect employee could keep their dayjob perfectly isolated from freelancing work, but in practice people with an hourly side hustle can't resist the temptation to sandbag at work.

The whole crux of the argument for shorter workdays and shorter workweeks is that people only have so many productive hours each week. Past the point of diminishing returns, their work quality and productivity drop. If someone is working 40 hour weeks, commuting, and also freelancing for another 10-30 hours per week, which job gets their best hours? Which job gets their least productive hours?

In my experience, side-hustle freelancers always let their dayjob work output slip, whether they expect to or not. This is why you need to watch employees very closely as soon as you get wind that they're freelancing.