Perhaps I'm confused about how we're using the term soft fork. My understanding is that this is where a new upgrade is designed to look valid to old nodes, and if you don't upgrade you rely on the miner majority to get you "back on track". For instance, P2SH was done this way - old nodes that didn't upgrade during that transition believed all spends of P2SH outputs were valid, even those spending someone elses coins.

In this case, the code you cite won't do anything because your client will never reject a block during a soft-forking upgrade, even if it does something that's supposed to be invalid or nonsensical.

If a new block version changes the serialization format or script language or SIGHASH rules such that old clients reject the block, then they will end up on a hard fork and the alerting code will trigger, which is correct and as it should be.