Cilent mod exploited bugs, giving players double attack speed.

It is possible for a human to replicate this without the use of the mod, albeit slower.

There is downtime when the land connects where the game puts you back to the idle animation. If you are holding the attack key, it'll look like one continuous loop. You can effectively reduce the down time by moving as soon as the move connects by simply pressing and releasing your movement key as fast as you can.

Lag compensation helps with this, which is what makes the mod attack extremely fast, it uses the animation cancellation to attack faster by pressing and releasing a movement key faster than most humans would be capable doing on top of having consistency.