Ghamidi Sahab Tafsir 61:6
The Quran does not claim that the name Ahmad was mentioned in the Bible, just that Jesus told his addressees this name.
It’s not an empty claim even if people were not able to record the name. Remember that the these 4 synoptic Gospels were written decades after Jesus passed away. The text is definitely adultered, even according to Bible scholars. Even if it was not, it was the life and message of Jesus being written from memory. The name may have been forgotten. Do you know Paul from Blogging Theology, by the way?