The use of "say" in the passage is another way of writing "for example" posing a hypothetical.
An example of this usage seen on English.SE - How to punctuate an example indicated by “say” which shows "parenthetical commas":
"If you have (say) a bucket..."
In the paragraph, "For instance" follows this sentence and an editor may find it wordy to use another clause construct that starts with "for". It is easier to modify the one in the middle of a sentence to "say" rather than the one that starts a sentence.
An identical meaning of the text:
Reducer are separate classes, the type parameters have different scopes, and the actual type argument of
KEYIN, for example, in the
Mapper may be different from the type of the type parameter of the same name (
KEYIN) in the
Reducer. For instance, in the maximum temperature example from earlier chapters,
KEYIN in replaced by
longWritaeble for the
Mapper and by
Text for the