This is correct. There will only ever be 21,000,000 BTC. There will also only ever be 2,100,000,000,000,000 satoshi making up that 21M BTC. The whitepaper is basically just saying that for those of us to stupid to deal in decimals, or more accurately us special americans who feel the need to fix everything that isn't broke in order to stroke some psychological sense of superiority over it, we can just move the decimal to wherever we want. However every time we do that and call that a BTC, the less divisible BTC will become. If 1 penny was equal to 1 satoshi then 1 full BTC would be worth $1,000,000 and obviously a market cap of $21T which I don't think is unreasonable by any means but there's going to be a lot of volatility most of the way there. None of this actually changes anything about BTC though, it simply adjusts our perception of it so it doesn't make us americans feel stupid. And of course there is somewhat of a psychological barrier for many people when it comes to owning 1 of something as opposed to .01 of something.