The game itself has an ESRB rating of Mature 17+ which should be enough of an indication that the video should have been age-restricted, the fact that he had to add his own censor bars means he was fully aware that the video was not appropriate for general consumption.
YouTube does not specifically state that censored nudity should immediately qualify a video for age-restriction but the several pages^[1][2]^ of guidelines heavily suggests that you should err on the side of caution.
Age-restricted videos are not shown in certain areas of YouTube. These videos may have limited or no ads monetization.
Guy was greedy, he didn't want to miss out on the ad revenue. When you have tied your livelihood to an account/service that uses automated checks and moderation, it is extremely irresponsible to push the boundaries, or even test them gently. YouTube is not YOUR job, you work for them, they own the platform, they set the rules.