The quote is out of context, so it isn't clear that we're talking about a non-upgraded node after a hard fork.  In that case, the non-upgraded node does not think the stronger chain is valid (because it breaks some consensus rule), so it will not accept it as the local best block chain.  The error/alertnotify is to tell you something is wrong.  For reference, that section of the docs was written based on the code is this function.

I'll push a minor revision to the quoted sentence so it's more clear and harder to take out of context.  Thanks!