Just to summ up / that attack would be like that:
A miner
1) modifies his own mining code to enter already the SW link into the correct place
2) mines a BAD block NOW, Long time before SW goes live
3) Now he can do lots of real txs like spending BTC (but get back when)
4) SW goes live and all blocks down to his BAD block get orphaned and his spending from 3) are neglegted
? correct ?
Soft-forks are not enforced on the older parts of the chain. Only in blocks from two weeks after 95% of the hash-power had signaled their intent to enforce (counted over a 2016 block interval after the change start-time).
For prior soft-forks, like strictder, there are many historical violations before where it was enforced. It does not make the chain invalid, only violations after the point where the new rule's enforcement begins would do so.