It's not an unethical hack, it actually helps the YT algorithm understand what category is the video part of. This works especially well in this scenario because kalari is a very narrow niche.
Tags do not boost your SEO at all, they rather help your video appear in the Recommended Videos section on the right of a YT video.
When I tagged competitors I was simply telling YT that it can show the videos to people watching another video from one of KalariLab's competitors.
There are 3 main ways to get views on YT when your channel is small:
1. SEO;
2. Homepage recommendations;
3. Recommended videos on other video pages.
The tactic with tags works only for the 3rd category above.
I hope that settles this discussion once and for all.
Sidenote: I still don't understand why this post was flagged...