Market cap is current price per coin times number of coins that exist. But if someone actually tried to sell a significant number of coins, the market price would quickly collapse. The money "holding up LTC" is not equal to its market cap. A better indicator of the money holding up LTC would be how much demand there is for LTC at or near its current pricepoint.
Exactly, the market cap is something people tend to get wrong. There aren't 6 billion USD invested in Bitcoin at the moment. It's less. If someone decided to sell a lot of Bitcoin, the price will go down, yes. I often don't get how people can be so obsessed with the market cap. On the other hand: If you see how much money there is in M1, M2, etc. you can only imagine what will happen if only a percent of that went into Bitcoin...