Even the fastest compressor cannot always catch the all impulses and peaks. Although its time parameters might be fast enough for this job from a technical point of view, the required settings and their effects would not correspond to the original intention of using a compressor in the first place.
The additional Soft Clip limiter of the alpha compressor plugin is specialized in catching short and transient-like signals reliably. The technical principle is different from a classic 'brickwall' design which completely forbids further level increases beyond a certain threshold.
Instead, it is working similar to an analog tape machine driving loud impulses into saturation, acting like a 'natural' limiter. Just as with tape, the characteristic saturation curve of the Soft Clip limiter results in rounding peaks instead of cutting them off.
It comes in very handy especially when the source material contains variable peak values. You will benefit from increased headroom and loudness, but your source material will stay musical because it will not be chopped by the limiter.