It can be heard that Zelensky demands a security guarantee from a 3rd country (like Germany, Turkey, France, Poland). So what does it mean. Suppose Poland agrees. Then does it mean that the next time Ukraine is attacked, Poland will join from Ukraine side. And how is this different from Budapest agreement
What Zelenskyy means is still unclear and still open to negotiation with Russia and the West. The Ukraine would like to have immediate NATO and EU membership, but NATO and EU have said that this is not going to happen anytime soon, and Zelenskyy seems to have concluded that asking time and again is not an effective negotiating strategy.
So Ukraine is now exploring "second-best solutions" which appear achievable, and tries to judge if they are sufficient for their needs.
- EU membership would come with security guarantees under Article 42.7, but they are less practiced than NATO Article 5.
- Promises from individual countries would have to be specific to be credible, and it would have to involve powerful countries close to Ukraine. No country is as close and militarily as powerful as Russia, so it would have to be a group. Poland would likely have to be part, and Germany, to assure logistics, but just those two might not deter Russia. The US is powerful, but they would need some others to help.
Yet if the assurances are too clear, they might be just as unacceptable to Russia as formal NATO membership, and if the guarantees are not part of a peace deal, it would mean the West going to war with Russia -- something President Biden has excluded several times. Perhaps the Russian military failures are so bad that President Putin looks for a face-saving exit, and "no NATO membership" might be this compromise even if the guarantees have the same substance. Or when the time for an agreement comes Russia still has a viable military position, and they can demand weaker guarantees as the price of their withdrawal.
Budapest agreement didn't commit any of the signatories to helping if Ukraine was attacked.
You could have Poland agreeing and then refusing to come to help Ukraine if they are attacked - but that would require breaking a treaty, and treaties tend to be honoured. Not all of them, but most of them.