> If you exhale a sufficient amount of air you can actually sit comfortably on the bottom of the pool without floating upwards.

this is not a universal truth.. depends on your body composition. If i exhale all my weight I will still have quite a few KG of positive boyancy.

> It's not a joke. I don't refer to drowning. It's just that the oxygen is in the blood, etc., not in extraneous reservoirs, making me heavy enough to sink.

It is because you dont have a giant bubble sitting in your lungs making you float yea.

> I think my trick uses the same basis as what the video explained.

We may be thinking of different videos then. In the video I watched the people who held their breath had a lung full of air, they didnt exhale. The guy did sit ont he bottom of the water despite having a lung full of air, but that is likely because he was either fairly muscular or had weights on.