Yes, fork-choice rules decide (protocol internal) which fork is the right one. "Fork" in this context means "version" or "branch" of a coherent decision tree. In the backend, there is nothing like a one-dimensional chain but rather a tree. Within Bitcoin, there are many alternative branches but only one is considered (within the protocol) as the canonical chain.

The decision for the "right" fork or branch (within the tree that is generated under one particular set of rules aka protocol) is based on a binary criterion that is not only decided by the longest chain but by the heaviest AND longest chain. In other words: the right chain within the set of protocol rules is always the longest chain with the most weight. Weight is entropy or energy and both can be translated into information (or state, so it is decidable).

But social legitimacy doesn't have to follow the protocol internal rules. And here comes the other meaning of "fork". Physics binds you to the rules within a protocol BUT physics does not bind you to the protocol. You can always pack your things and move to YOUR fork of choice even though it was not considered as the correct chain under the previous set of rules. A better word for this would be community-fork. So, not every fork is a blockchain. A fork becomes a blockchain when it gets implemented into a new set of rules.