The description reads “Send your enemies flying with an invisible blast of air!”, so I’m pretty sure ‘Atmo’ refers to ‘Atmosphere’ rather than a misspelling of ‘atom’
Hate to be a nitpick, but I believe this name change was incorrect and should be reverted back to “Atmoblaster 777”. The item got renamed today, but as someone stated above the name was referring to atmosphere as the gun is centered around sending gusts of air and doesn’t have any references to atoms.
The tooltip also mentions compression levels which feels more as a reference to air pressure.