I would put money on it being a 3D printed add-on - probably with a cheap laser pointer on the inside if it functions at all.
Infinity Ward artists designed a bunch of stuff for the 'new' Modern Warfare that would not fall foul of trademarks and trade-dress litigation (hence the moniker M13 for the gun), and this laser is one of those creative licenses - in this case roughly based on a Steiner DBAL-I2 unit
This person's YouTube channel is about MW guns and 'real life' remakes of the reload animations. Makes sense that he has someone model and print the 'Tac Laser' from the game to spice up his video.
Source:
https://www.artstation.com/artwork/xzXAJY
CG renders of the 'hero' gun:
View attachment 94367
View attachment 94368
Note, on both the YouTube video, the picture on his YT banner, and the render above - there seems to be no way to fit batteries without removing the laser - which IRL would suck for zero-repeatability.