Now i never said you can gain more than 10k points. I what I said that is if you hit 10k and then go below it again, you can work your way back up. The only way to stay at 10k though, is not to falter from your alignment.
For example:
I am at 10,000 Dark Side
I do a Diplomacy that gives me 75 Light Side, 10,000 - 75 = 9925 Dark Side ( I drop to Rank 4 )
I do a Flashpoint that gives me 100 Dark Side, which takes me back up to the 10,000 DS cap and gives me Rank 5 again.