Earlier quoted context omitted.
Just because I can buy a bitcoin from somebody for $1000 a few months ago doesn't mean that same person is willing to buy it back from me for $1000 dollars today. The value in that bitcoin has evaporated, disappeared, gone up in smoke. Bitcoins are only worth how much people are willing to pay for them; what was spent on them in the past does not convey to them any value today.
The value hasn't evaporated - if you bought a bitcoin for $1000 a few months ago and now you can sell it for $100, then value of ~$900 (assuming that 'real worth' of USD didn't change much) was transferrend from you to the guy who sold you that bitcoin back then.
Think of it this way, I buy a nice car for $100k. The person selling it gets $100k in cash, and I get $100k worth of car. A year later, I crash the car and it is now worth $1k in scrap metal. Where did my $99k worth of value in the car go? Did the dealer somehow acquire that value? No, I simply destroyed it. I destroyed the value of the car, it wasn't taken from me. The value of the cash I gave the dealer? That is more or less still the same, but that is his business, not mine.
Value is not zero sum. Creating and destroying value is trivial.