LTNman
The relationship between runway length and declared distance may not be obvious because it is not one of physical dimensions. It is legislative, or, one might even say, bureaucratic.
SEN is Code 3C status by virtue of its less than standard runway width. This brings in the other requirements of a Code 3C airport, and amongst these is a maximum TODA of 1799m. The total paved runway surface at SEN is 1991m so one would expect the "real" TODA to be something in this region (unless of course there are obstacles outside the airfield boundary that cause it to be reduced - a close evaluation of the airfield data would be needed).
I believe that national authorities have the authority to grant a 10% increase though I seem to recall a discussion on pprune a couple of years ago where it was suggested the UK CAA are unwilling to permit such alleviations. As a point of interest the original quoted distance of the paved surface was 1979m, which is exactly 1799m plus 10%.
As regards the caution on the airport website, I suspect it is no more than the airport covering itself. I find it hard to believe any code C aircraft would have a problem at SEN. Remember that it has been used for years for maintenance purposes by A321, 738 and 757, the latter being Code D.