That's because it's just wrong, like everything from the docs of ledger and its descendants (and every "accounting for developers" article, except surprisingly not TFA here!), and accountants are just correct to use unsigned numbers.

A credit entry is a source, and a debit entry is a sink. When I do $4000 worth of consulting services work for Joe Bloggs, this gets entered as a $4000 credit to Income - Consulting Services (the source of the flow of funds), and a $4000 debit to Accounts Receivable (the destination of the flow of funds). Income, here, represents the outside world from the perspective of my business. Later, the receivable becomes cash when Mr Bloggs writes me a check, so I credit Accounts Receivable $4000 and debit cash (actually something like Assets - Coolbank Business Checking) $4000.