1. Satoshi explicitly said "20 hours" in the quote I gave you.
2. Since 2009, we have 10 minutes per block, that means 1200 minutes per 120 blocks, which means 20 hours.
3. In the older version, you have 15 minutes per block, instead of 10. And then, 100 blocks means 1500 minutes, which means 25 hours. Then, if you change 15 minutes into 10 minutes, you changed also 100 blocks from 1500 minutes into 1000 minutes. And that could be too short, so Satoshi added 20 blocks to land in a "nice, round number in between". If you replace 1000 minutes into hours, it will give you 16 hours and 40 minutes. Not a "nice, round number". If you calculate it for 1500 minutes, it is "25 hours", but it was longer than 24 hours, so probably a bit too long. And then, "1200 minutes = 20 hours = 120 blocks" was just a nice value between 1000 and 1500 minutes.

Edit: Also remember that Satoshi loved decimal numbers. So, which values would you pick, that would meet those conditions?

1. The number of minutes, divisible by 10, or even by 100 if possible.
2. The number of hours, divisible by 10 if possible.
3. The number of blocks, also divisible by 10 if possible.

There are not that many values you can pick, if you replace "15 minutes per block" with "10 minutes per block", which means "100 blocks = 1000 minutes", instead of "100 blocks = 1500 minutes".