If Satoshi traded his (according to your example) $10 billion worth of bitcoin with Greeks for their property, they would not be any poorer because they'd have all that bitcoin in exchange. They might even be better off since by entering into the transactions they would have increased bitcoin's adoption and hence its value.
Yours is a false argument because your conclusion assumes bitcoin is worthless, which assumption is inconsistent with your premise that bitcoin is circulating with a value of $10,000/BTC.
Bitcoins (or any currency) are not wealth, they are tokens that people can exchange for wealth with other people.
When you are measuring the wealth of one person or company, it is correct to include any currency that they own, because currency is so easily exchanged for other wealth with other people or companies.
But when you are measuring the wealth of a country or of a planet, you cannot include the currency that its inhabitants own, unless they can easily exchange it all with people outside the country or planet.
In that example, before Satoshi spent his stash, the Greeks owned a lot of houses and land, and 9 million bitcoins. After Satoshi's buying spree, they would own a lot fewer houses and land, and 10 million bitcoins. Unless they can use that 1 M bitcoins to buy 10 billion USD worth of stuff from people outside Greece, they will be poorer by 10 billion USD.
Wealth comprises those assets people value and people can value bitcoin just as they can value other currencies, gold, art, houses or anything else.
It is not necessary that in order to include their currency in their measure of wealth people must be able easily to exchange all their currency with people outside their country or planet. It is only necessary that at the margin they can exchange a part of their currency for something they value more highly: currency has value as a means of exchange and it does not matter with whom they exchange it. A more freely exchangeable currency will, all other things being equal, clearly be superior (more valuable) than a less freely exchangeable currency, but the idea that the whole currency must be exchangeable with people outside the country or planet is false: it is enough that you can find one person willing to transact with you.
If Greeks choose to exchange property for $10 billion of bitcoin they are not obviously poorer by $10 billion, unless you presuppose no one wishes to exchange anything for bitcoin any more so soon after it becoming worth $10,000/BTC. There is no obvious reason to make that supposition; it would be an arbitrary assumption. Apart from anything else, there is no obvious reason why bitcoin could not become a means of exchange for dealing with Greeks if Greeks valued them.
