If you're educating people about bitcoin, maybe have a cute little graph on how many bitcoins were mined in the first 4 year period before the first halving and show the math in a visual way and how it will never reach 21 Million.
21 million is a rounded number. Is it a serious problem with 21 million BTC and 209999999.99 BTC. It is not a serious issue but I agree with you that a note for it is fine.
The 21 million figure can be used with a note and reference to 209999999.99 BTC as total supply. I remembered this issue was discussed in the past.
I could be wrong, but I get the impression that he didn't choose 21 million. He chose 50BTC as a block reward and then chose to cut that reward in half every 210,000 blocks. If you do the math, that just happens to work out to a bit less than 21 million (20999999.97690000 to be exact).
4 years for each halving is another rounded figure. BlackHatCoiner explained it above.
https://www.bitcoinblockhalf.com/Past halving event dates
The first halving event occurred on the 28th of November, 2012 (UTC) at block height 210,000
The second halving event occurred on the 9th of July, 2016 (UTC) at block height 420,000
The third halving event occurred on the 11th of May, 2020 (UTC) at block height 630,000