There's no way to get this item in-game and it's never going to be added back in again. Information is definitely out-dated, and you shouldn't really take it to heart.
"Holo scope, not made out of real holograms."
The Holo was an optical attachment for the MP5 and the M4A1 that was used mainly for testing purposes.
It added a small sights to the top of the gun which you would then see through when you ADS'ed.
While L-Brain Sidework was still getting their bearings, an employee proposed that they also produce Attachments to go along with the guns that they produced.
The Holo was one of their first attempts. While it did serve as a cool accessory, not that many people found that it was useful and it quickly grew pointless to even buy it. The product was soon discontinued, but not before using the little budget L-Brain Sidework had from sales to try again.
This is the first Attachment you had to pay for in R2DA.
This also became the first Attachment to have Custom Sightmarks with the RedDot variation available for 500$.