The other thing the CEO mentioned in the email was mining industry unions - this is the prevalent view on unions in the eyes of non-unionized workforce in the country. This might be propaganda, but ask a Polish person what do they think of the coal miners and you will hear some expletives. It's common knowledge that the coal industry has long been unsustainable and the only reason it's being kept up is to gain some votes from that part of workforce.
An additional thing to know here is that the largest trade union is Solidarity, the direct successor to the original Solidarity trade union turned political movement which was the dominant force driving the democratic changes in the 80s. The movement is very politicized and heavily leans national-conservative (e.g. the official union newspaper basically reprints unfounded news and conspiracy theories), which further deepens societal divisions on the topic.
I have the feeling that a union this guy formed aimed to cut down the fee for any new joiners or people not aware how much they were being ripped off.
In Poland there's a strong labor law, but there are also "escape hatches" like working on a B2B contract (you create a 1-person company and provide services to your employer - in reality you work just the same as before but you pay less taxes and have only the safety net you negotiate for yourself - usually much less than on a regular contract). It's very common for people earning more than ~15 000 PLN per month cause taxes are much lower then, and the market is so good that abuse isn't likely anyway (people can always just switch jobs if they dislike something).
Even on B2B you still get free healthcare, free education for your kids up to the university level, state retirement, etc. But you probably pay minimal possible retirement and healthcare taxes (that's the point of working on B2B) - so you will get minimal possible retirement and your sick days will be paid as if you worked on minimal salary even if you earn 5 times that when you're not sick.
Despite no safety net and little regulation - it's still very common to take long vacations, sick days, maternity leave etc on B2B - simply because it's the norm in the society. People just take a hit (you earn more so you can afford it).
I've never heard about people creating trade unions in IT in Poland, it's like a setup for a joke. Why bother when you get 10 better offers on LinkedIn every month - just change the job. But I guess if you work on a regular contract and want to for ideological reasons or whatever - you should be able to start a trade union, so this particular employer will get f**d in the court.
BTW for other industries the situation is much less rosy - in some cases people were pressured into working a B2B contract (or else we won't hire you) with low salaries (~3000 PLN per month) and then it just sucks with no upside. But I don't think it's that common, big companies usually don't bother cause it risks problems with law.