I read, 80% of the coins being dumped now are from long term holders, they want OUT!
There are more sellers but who will buy or mine Bitcoin at such high prices
I think what you heard might be wrong. If it was so then there would have been a massive dump and the price would have gone way more down that it has gone now. This might be just another temporary correction and we should probably wait another week or two to see whether the market gets stable or not.
If you see the history of BTC you can see that the same pattern repeats again and again. Once the wave has completed another wave starts and the prices starts shooting up. Just wait for the right time.
Exactly if 80% of the coins were to be dumped then the rates would have been affected highly due to the supply being more then the demand but that not being the case whatever he said can be neglected, but also the fact that bitcoins fall from 20k$ to 6k$ means at some point there was a panic selling of bitcoins which indeed caused the downfall of the coin to such a rate, whatever be the reasons, bitcoin trading wont end anytime soon.