Let's have a look at this new sentence §23 Absatz 1 Nr. 2 Satz 4 EStG:
"4Bei Wirtschaftsgütern im Sinne von Satz 1, aus deren Nutzung als Einkunftsquelle zumindest in einem Kalenderjahr Einkünfte erzielt werden, erhöht sich der Zeitraum auf zehn Jahre."
As Wirtschaftsgut (which is just tax-speak for a "thing"), you have the foreign currency (= Fremdwährung) - in Jasper's case $.
So, for this sentence to apply to that foreign currency, and for the speculation period to be prolonged to 10 years, the currency itself needs to have created income (= Einkünfte). This will only be the case if that account was interest bearing.
Buying a stock/fund with a foreign currency doesn't mean that the profit from the sale of the foreign stock/fund suddenly falls under §23 EStG "Private Veräußerungsgeschäfte" which according to §22 Nr. 2 EStG are "sonstige Einkünfte" as defined in §2 Absatz 1 Satz 1 Nr. 7 EStG.
Only the earnings from foreign currency itself are "sonstige Einkünfte" and would have to be taxed at Jasper's personal, variable income tax rate (not at the 25% Abgeltungsteuer flat rate)
Profit from selling stocks/funds falls under §20 Absatz 2 Nr. 1 EStG, i.e. it is capital income ("Einkünfte aus Kapitalvermögen") as defined in §2 Absatz 1 Satz 1 Nr. 5 EStG, and is taxed with the Abgeltungsteuer of 25%+Soli (except if he owns 1% or more of a company, then it's taxed differently, see my post of 23. August 2016).
This position is also taken by the BFH in its ruling of 21.1.2014 BStBl. II S. 385 (which is also mentioned in the Einkommensteuer-Hinweise, which are part of Einkommensteuer-Richtlinien, under "Fremdwährungsgeschäfte", so it's known to the Finanzamt and it has to apply this ruling), in which they say that selling a stock/fund and getting an amount in a foreign currency for it means:
that stocks/funds are sold (this falls under §20 Absatz 2 Nr. 1 = Einkünfte aus Kapitalvermögen), and, that at the same time,
the amount in foreign currency that you get for said stock/fund is "bought" (this being a foreign currency transaction, it falls under §23 Absatz 1 Nr. 2 EStG).
So, as long as:
Jasper's foreign currency account isn't in itself interest-bearing, and
he didn't reuse that specific tranche of his $ within the normal speculation period of 1 year for buying another stock/fund (which would be unlikely, since they assume a FIFO - first-in-first-out - order of using up the money, see §23 Absatz 1 Nr. 2 Satz 3 EStG so he would have first needed to have spent all his "older" $ in order to reach that specific tranche of his money)
then what I told Jasper on 28.9.2016 further up in the thread holds true.
Here's a nice article with an example showing a case where the above condition 2 isn't fulfilled, it's a non-interest-bearing $ account (condition 1 is fulfilled), but the owner makes the mistake of trading at intervals of less than a year and of re-using that same amount of money each time (well, he doesn't have any choice, since he always uses the whole amount of $ he owns).