A soft fork without consensus does *not* lead to a (lasting) chain split if and when a majority of hash power mines the soft fork chain.

The non-soft fork chain will be re-org'ed out of existence every time the soft fork chain becomes longer, because non-upgraded nodes will switch to it.

If users/miners on the non-soft fork chain want to prevent this, they need to take action to reject the soft fork chain.

Luke is right about that.