A lower-ranked witness not running the softfork code could have produced a block that contained such transactions. But the witnesses running the softfork would have rejected that block itself (they wouldn't have added it to their local chain of blocks).
This would have resulted in two different chains on the network: one with the block and one without. These two chains would then compete and eventually the longest chain wins (which would have been the softfork chain, because far more active witnesses were running the softfork).
So the block would have become "orphaned" in the terminology of blockchains and the transaction would not be recognized as valid by the network.