Japanese workers turn to resignation agencies

Can anyone in Japan share what ground truth looks like around this? Does this churn matter to businesses when they’re in a labor supply shortage? Do these folks have other jobs they’re moving to? Or are they potentially NEETs bailing on being employed?

This definitely exists, even when switching jobs.

However I would say that IMO it's another case of foreigners buzzing by depicting boring and common stuff under a "weird Japan" light.

Shitty companies manipulating employees to stop them from resigning is something that exists in any country. And this escalating to the labor authorities or going through a lawyer is not a rare thing anywhere either.

It happens more frequently in Japan because the culture of not being confrontational is strong. The fact that lawyers can afford to specialize in this matter alone is just a logical result of the larger number of customers.

EDIT: I also want to add this: if you have been in a company for a while, you are eventually going to see or hear about how resignation is handled for other employees. If you want to quit and already know that the company is going to harrass you and make your life hell, is it so weird to save your time and mental health to delegate all of that to a dedicated professional?

Does it? You hear stories in the US of people trying to manipulate employees into not quitting or lambasting them for leaving, but trying to actually, seriously deny their ability to quit is nearly unheard of.