IW's site does not (or at any rate did not last time I checked) achieve complete coverage as he is still in the process of transcribing the microfilms. Further, as the original records were handwritten, there can be errors of transcription.
All fatal accidents in working mines were recorded in the period you are concerned with - so even if it was a single fatality, it would have been recorded. After detailed annual reports of H.M. Inspectors of Mines resumed after the interruption of WW1, not all inspectors listed names - some only published statistics. I am unsure whether Kent has detailed lists or only statistics for this period.
I have just posted on another thread on this site an explanation of the relationship of IW's data to the inspectors' published reports.
The death certificate will state the verdict of the coroner's jury as the cause of death - e.g. 'accidental death' (rather than for example, 'fall of roof'). In this period, local newspaper reports generally provide the most detail - and also coroners' records if they survive.