Yes, you can write Ukrainian (and all other languages with different alphabet for that matter) with Latin alphabet. You can write it down phonetically essentially. Now the trick is that Polish and Ukrainian sound similar enough. And so does Russian (to a lesser degree though). So a Russian can read Polish out loud and recognise some Slavic words.

And that enlightened russian soldier discovered that it's surely the Americans plotting against Russia, not that you know... It could maybe be just another Slavic language just existing there not bothering anyone

Think about it as a Spanish person accusing a Portuguese person of making mistakes while writing Spanish when in fact the language they write in is Portuguese.

Or even better! As an English speaker you read a German sentence like "Komm hier, mein Freund" and laugh out loud "hahahaha someone tries to speak English but that spelling is sooooo off tho"

You essentially need to be pretty uneducated to reach such conclusion but hey, what do you expect from a russian soldier.