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The A[$1] before the code block: A[$1]{$5=A[$1];print}

nulls and blanks are considered false and won't run the code, anything else is true and will run the code block.

I think my answer is correct but you have the files on the command line in the wrong order.

awk 'NR==FNR{A[$1]=$2;next} A[$1]{$5=A[$1];print}' file1 file2