Information is definitely out-dated, and you shouldn't really take it to heart.
The Mega Knife was one of the first three attachments in R2DA.
It served no purpose at all other than to serve the purpose of testing the whole system out.
Theoretically, this would add a knife to the MP5 and allow the user to do a charge on zombies of sorts.
Attachments weren't initially considered to be mass produced by manufacturers. Survivors were upset by this decision.
Being rather passive-agressive, survivors dreamt up some things they would've liked to put on their guns, and the Mega Knife would become one of the more popular ones. Ultimately, the sarcastic tones got to the manufacturers and they went to work.