He immediately told his boss, but he did not find any solution.

I also know (informally) that some big boss asked why there is such a big attrition rate in that department (generally the company culture is great and the attrition rate is quite small), so managers are struggling to keep all folks in place.

scheduled a few other 1:1 meetings to mention that I must say what I want so they can try to improvise something and stay in the company

This looks like everyone KNOWS the problems and reasons behind them but deny they exist and start "digging" with each new occurence of profits from those problems. Trating them with suprise and EXPECTING you to provide solutions.

I do not fully understand what they are trying to build there

It's THEIR responsibility to explain that. Especially when such understandig would help you stay. I assume hey didn't do that.

They imagined I was coming from a "worse place" and there would be no problems when working in the department (that's why they skipped me when having 1:1 discussions with each team member)

"I thought you know" - so they assumed, without fact checking that they "promote" you somehow? That the "Bad place" is better because you're coming from "worse" one? That's a book example of manipulation by gratitude. They give you so much and that's how you repay them?

For me to work in the Good Place, it would that a colleague from there to no longer there which is impossible (I never asked for that anyway).

Another example of manipulation. They are blackmailing you "If we hire you it mean you will be responsible for firing someone else".

She managed to convince lots of folks to stay including in the very last day

I think she managed to manipulate people into staying. Not convincing them. That's why they bring her to talk with you. She's probably company No.1 manipulator.

Your boss is doing "give nothing, take everything". Promises are cheap and worth the paper they are written on. Especially so vague as the ones made to you.

I would say:

We talked about my position and responsibilities before any transitions were done. I was exluded from any of those and only informed about the results. I don't see anything we can discuss as, I hope, you are aware that there are multiple problems on the company side that I refuse to solve by my sacrifice of pay, happiness, pleasure of doing my job and overall mental stability. It would be nice if you could acknowledge that NOW is bad moment to promise me anything and stop talking with me about that.

He immediately told his boss, but he did not find any solution.

I also know (informally) that some big boss asked why there is such a big attrition rate in that department (generally the company culture is great and the attrition rate is quite small), so managers are struggling to keep all folks in place.

scheduled a few other 1:1 meetings to mention that I must say what I want so they can try to improvise something and stay in the company

This looks like everyone KNOWS the problems and reasons behind them but deny they exist and start "digging" with each new occurrence of profits from those problems, treating them with surprise and EXPECTING you to provide solutions.

I do not fully understand what they are trying to build there

It's THEIR responsibility to explain that. Especially when such understanding would help you stay. I assume they didn't do that.

They imagined I was coming from a "worse place" and there would be no problems when working in the department (that's why they skipped me when having 1:1 discussions with each team member)

"I thought you know" - so they assumed, without fact-checking that they "promote" you somehow? That the "Bad place" is better because you're coming from a "worse" one? That's a textbook example of manipulation by gratitude. They give you so much and that's how you repay them?

For me to work in the Good Place, it would that a colleague from there to no longer there which is impossible (I never asked for that anyway).

That's another example of manipulation. They are blackmailing you "If we hire you, it means you will be responsible for firing someone else".

She managed to convince lots of folks to stay including in the very last day

I think she managed to manipulate people into staying. Not convincing them. That's why they bring her to talk with you. She's probably company No.1 manipulator.

Your boss is doing "give nothing, take everything". Promises are cheap and worth the paper they are written on. Especially so vague as the ones made to you.

I would say:

We talked about my position and responsibilities before any transitions were done. I was excluded from any of those and only informed about the results. I don't see anything we can discuss as, I hope, you are aware that there are multiple problems on the company side that I refuse to solve by my sacrifice of pay, happiness, pleasure of doing my job, and overall mental stability. It would be nice if you could acknowledge that NOW is a bad moment to promise me anything and stop talking with me about that.