That's the 8 Pro he tested, not the 8. Towards the end of the video a screen shows it to have "14.9G" of RAM. The Pro comes in 16GB and 24GB models (non-Pro in 12 and 16), so I gather that his test phone is the 16GB ROG Phone 8 Pro, which actually does *not* come with 16GiB of RAM, but rather the base-10 16GB which amounts to 14.9 base-2 GiB.
Correct me if I'm wrong in that interpretation. I am a bit surprised, and also disappointed, to notice that phone manufacturers have taken the HDD manufacturer technique of measuring RAM in base-10 gigabytes to undersell consumers, as opposed to the base-2 counts that we normally expect.