The video was released right after a recent Season 4 update, which saw numerous balancing changes and other small fixes. What WhosImmortal claims however, is that there were some buffs added to Call of Duty: Vanguard attachments that were not listed in the patch notes.
WhosImmortal also revealed that some perks also received a hidden buff, and that players need to take advantage of the hidden improvements to help dominate their competition.
In his video, WhosImmortal claims that the Hand Stop underbarrel is no longer the top option for the meta, and that players can find success with many different underbarrells, it is just dependent on the gun.
The term "secret buff" gets thrown around by many content creators and pro players, but what does it really mean? Most secret buffs are benefits an attachment brings, but is not listed in the patch notes. An example of this is the Disable perk. The perk initially is supposed to slow movement by hitting players in the legs. However, the Disable perk actually increases muzzle velocity.
Players can use Sym GG to discover which perks have secret buffs and how attachments affect different weapons.