I believe they are talking about 30 day gametime included but thanks for letting me know! I'll do another video anytime soon trying to do it once again and test it out also on US (technically should work if you buy EU version and use it on US so will try it out) but i don't recommend trying it...
Buy Battlechest and Legion from sites like kinguin or G2A (goes like 40$ combined)
Redeem Battlechest and Legion on your main account so you'll get another license
As all services (including boost you get free with copy of legion) are account wide and not limited to license, you'll recieve boost...