This excerpt comes from lines 138-139 of chapter VIII of the 2003 edition of Lingua latina per se illustrata:
Quis saccum portat? Servus saccum portat. Quī servus? Servus quī saccum portat est Syrus. Is/ille servus saccum portat.
In the right hand side of the page, there is the following explanation:
is servus = ille servus
I believe there is an error in such explanation and this should be
is = ille servus
because is is a pronoun that can indeed be replaced by ille servus. Is my interpretation correct or am I missing something?