I think it is important to know what "predefined" means in the context of compiling the data step code. Here, it does not mean "defined previously in the ARRAY statement", but "defined previously before the statement was encountered".
It needs familiarity with how the data step compiler builds the PDV to not be misled by this particular sentence. This familiarity grows out of experience, but there is also Maxim 41. Most experience arises from mistakes.
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