SSD
Precedent is often but not always the first to do or be recognised for doing something.
Autoland really would only work in a Trident because it had triplex systems all over it and other airliners of the time didn't. Several didn't have Triplex hydraulics or power generation and you cannot practically add those systems back into an airframe that was not built around them.
So for all its many faults the dear old DH/HS 121 gripper did indeed set the precedent for autolands. Sure many followed but Trident was the first to do it reliably and in full airline service.
Precedent means an earlier event or action that is regarded as an example or guide to be considered in subsequent similar circumstances. So, many later airliners had autoland and they had it because the trident provided a practical and realistic guide on how to land a plane automatically and that's why the Trident is the one that set the precedent