TL:DW; Some guns allow much faster strafing left and right while zoomed in than others, in the video the two examples are the F2000 and K2002, although there are probably others.
This doesn't seem to have come up before, and it's not on the charts. We have a lot of data on guns but not on player mechanics.
Another interesting thing to note from that video is he holds Ctrl when jumping from a height which would damage, in order to avoid the damage. Not sure if it eliminates all damage, probably just reduces it some. Will have to experiment with this myself.
with the first reference for firing from the hip and the second for ADS. In the second case this then leads to weapons\Common\ZoomLevels\FastIronSights which contains "MoveSpeedMultiplier 0.75" among other things.