This scripts finds all files in given directory tree, reads their com.apple.metadata:kMDItemDownloadedDate xattr and changes last modification date to that value if exists.

touch_them.sh

#!/usr/bin/env bash

set -e

set_date_attr() {
    local hex
    hex=$(xattr -px "com.apple.metadata:kMDItemDownloadedDate" "$1")
    if [[ $? = 0 ]]; then
        local date_xml
        date_xml=$(xxd -r -p <<< "$hex" | plutil -extract 0 xml1 -o - - | sed '4q;d' | sed -e 's/<[^>]*>//g')
        echo -n "file '$1' has kMDItemDownloadedDate $date_xml"
        local date_touch
        date_touch=$(date -jf "%Y-%m-%dT%H:%M:%SZ" "$date_xml" "+%Y%m%d%H%M.%S")
        echo " > touch -t \"$date_touch\" \"$1\""
        touch -t "$date_touch" "$1"
    else
        echo "file '$1' does not have kMDItemDownloadedDate xattr"
    fi
}
export -f set_date_attr

cd "$1"
find . -type f | xargs -I {} bash -c 'set_date_attr "{}"'