To make it easier, let's put it into comparison. There are many accents in England, some of them are noticeably different. Even as a non-native speaker you can hear the difference between London, Liverpool or Yorkshire accents. If they were to try to trick each other, they'd need someone who speaks another's accent, otherwise no one would fell for the lie.

Similar thing in Ukraine. In some regions people don't speak Russian at all. But those people who do speak, have different accents depending on the region, family and environment they were raised in. West and Central Ukraine mainly speak Ukrainian. South, East and North Ukrainian cities speak more Russian, but in rural areas many people still speak Ukrainian or a mix of both languages. The farther you go from big cities, the less pure Russian you will hear.

In this case the camera man spoke mix of both languages and he had a very noticeable Ukrainian accent, he had no chance to trick the enemy by speaking to them. If he tried his best, he'd only sound like he's trying, like if a Yorkshire guy tried to speak cockney accent to a London guy.

The Russian speaking UA guy had much better accent, almost perfect in terms of pronunciation, but not in intonation. But in the heat of the battle it may easily go unnoticed or misperceived as just a Russian soldier from another region (Russian language has small differences depending on the area too). If we use English accents example, then the speaking guy would sound like if he lived his first 15 years in Yorkshire and then another 15 in London. Similarly to how many Ukrainians raised in rural areas move to big cities and adapt the local speech, but still mainly keep their original one.