Indeed, it is not known whether there are other Bitpay incoming addresses that are not included in that "wallet".

The site that tracks the "BitPay input wallet" scans the blockchain for transactions that have two or more inputs, and assumes that all the input addresses of each transaction must all belong to the same person or company.  That criterion defines clusters of addresses with same owner, that the site calls "wallets".

Periodically, Bitpay merges all the amounts that it received from customers and sends them to Bitstamp or other buyers.  The site sees those transactiions, and then identifies all the input addresses as belonging to the same owner.  Then it  is easy to tell that the owner is BitPay.  

That criterion may indeed miss some input address that is owned by Bitpay but is never combined with other known Bitpay addresses in any transaction.  However, just by looking at the addresses that were identified as Bitpays, we get 1000 to 2000 BTC per day, which matches the "1 million USD per day" that BitPay claimed to process in May 2014.   One can also find in that wallet some famous payments, such as the 0.5 M$ house bought by Josh Zerlan of BFL  and the 1 M$ downpayment by HashTrade(?) to BFL.  So, that "wallet" does seem to include most of what BitPay processes.

EDIT: The CoinJoin anonymizing service combines input addresses from different people in the same transaction, and therefore breaks that clustering criterion.  There is a huge "wallet" called "MtGOX and others"  that seems to comprise all addresses that were once used in a CoinJoin transaction.