There is no such thing as an illegal invasion.
An invasion is, well, an invasion. We might pretend that we can legally invade a county by putting up a suitable PR fig leaf (e.g. Iraq and the "Weapons of Mass Destruction" BS) but an invasion is still an invasion.
If you carry this 'illegal invasion' rhetoric too far you'll end up justifying the Russian "Special Military Operation". It might have flown under a lot of peoples' radars but before this "SMO" there was actually a rather nasty civil war going on in the east of Ukraine. The war was caused by a refusal of significant sections of the country to agree to what was effectively a coup d'etat in Ukraine in 2014. The war had caused a significant number of deaths, a lot of displacement and general destruction the government of Ukraine had shown no signs of wanting to figure a way to stop this short of crushing the local populace (i.e. killing and destroying a lot more of the area). Given the proximity to the Russian border this alone would justify the so-called "SMO".
But I suppose its only OK if we're doing it.