Quality saddle and seat post
100kg is not super heavy and  well within the range of rider weights that bicycles and bike components are designed for. A decent quality seat and seat post should not break underneath you. 

Some saddles probably do have a specified rider weight limit, in the same way the most bicycles do. I quickly looked for some information and found that Fi'zi:k actually explicitly states they have no rider weight limit on any of their products. I also looked at WTB but have found no information at this time.

Looking at the picture you supplied, and the description of what happened, I may be able to offer some insight. The seat in the picture is positioned very far back. If the seat that broke was positioned in the same way, your weight would have been acting behind the seatpost and trying to rotate it backwards rather than pushing straight down on the rails and post. You may have also over-tightened clamping bolt. These two factors would have over-stressed the bolt and caused it to snap.

I guess you fell off the back of the bike behind the seatpost, which would have been fortunate as you avoided being impaled on the post as well as breaking your leg.