When trading Forex each currency is traded relative to another. So when shorting a currency you must go long another currency vs the currency you are shorting, it seems a little odd and can be a bit confusing, but here is the explanation that Wikipedia provides:
An example of this is as follows: Let us say a trader wants to trade
with the US dollar and the Indian rupee currencies. Assume that the
current market rate is USD 1 to Rs.50 and the trader borrows Rs.100.
With this, he buys USD 2. If the next day, the conversion rate becomes
USD 1 to Rs.51, then the trader sells his USD 2 and gets Rs.102. He
returns Rs.100 and keeps the Rs.2 profit (minus fees).
So in this example the trader is shorting the rupee vs the dollar.
Does this article add up all other currency crosses to get the 'net' figure?
So they don't care what it is depreciating against?
This data is called the Commitment of Traders (COT) which is issued by the Commodity Futures Trading Commission (CFTC)
In the WSJ article it is actually referring to Forex Futures.
In an another article from CountingPips it explains a bit clearer as to how a news organization comes up with these type of numbers.
according to the CFTC COT data and calculations by Reuters which calculates the dollar positions against the euro, British pound, Japanese yen, Australian dollar, Canadian dollar and the Swiss franc.
So this article is not talking about futures but it does tell us they got data from the COT and in addition Reuters added additional calculations from adding up "X" currency positions.
No subscription needed: Speculators Pile Up Largest Net Dollar Long Position Since June 2010 - CFTC
Here is some additional reading on the topic if you're interested:
CFTC Commitment of the Traders Data – COT Report
FOREX : What Is It And How Does It Work?
Futures vs. Forex Options
Forex - Wiki