Russia also loudly denied any plans to invade Ukraine until they rolled across the border. And they've been repeatedly caught committing barbaric atrocities against civilians in occupied territories.
So if the Poles want to sign up for military training, then more power to them. Being willing to defend your loved ones and your country is a noble endeavor. You don't even need a specific reason. And if the Poles want the ability to defend themselves without being utterly reliant on their NATO allies, that's their right.
Whenever I dug in more deeply, Russia has warned about using nuclear weapons to preserve its own territorial integrity.
One could argue that declaring Donbass Russian is a semantic trick to "justify" using nuclear weapons in Ukraine. But where is the threat against EU countries?
The Russian ambassador to London has just declared that they won't use nuclear weapons in Ukraine (Sky News). Is he lying? Possibly, but that contradicts open threats.
A 300k active service military would be of course nothing to scoff at, especially with a lot of long range weapons.
But what we see in Ukraine is they had biggest army in Europe, and they also wanted to go mostly professional, but their core force, the most well trained soldier took an enormous hit, and is melting away.
Soldiers whom they spent years preparing are still humans, and are dying to artillery, air strikes, and zerg rushes of Russian "disposables" (draftees, Wanger mercenaries, and LPR/DPR levy.) These highly trained soldiers are not trading their lives meaningfully when they sit in trenches, and can't attack because the frontline is so stretched out, and manpower is drained for menial tasks.
This is all shows that you can't fight something like Russia with professional army only if you are a country of only 44m people.
A degree of differentiation in between career warriors who will spend a decade or more in the military vs. somebody who will go for a 2-3 year contract, and reservists with minimal training is absolutely essential.
Israel maintains a 1 to 3 split in between professional soldiers, and the good part of their reserve (people who regularly train, fit, and had good scores in tests.) The ratio is way bigger on the paper though.
We now see that Russia has difficulties getting draftees to fight abroad, so it is hard to imagine that Poland is at any particular risk right now.