Witness inflation is done on the joint transaction itself. Unconfirmed ancestors are not needed for this attack vector.
It has the same effect though: the inflated transaction is thus paying a lower feerate than expected, which makes it easy to replace with a transaction that pays the expected feerate (when not coupled with descendant pinning).
It is also hard to reliably pull out: if you also pin with descendants, that means the descendants will be even lower feerate and will thus be evicted from some nodes’ mempool, allowing a replacement to propagate. It’s impossible to quantify that though, since it really depends on the topology of the whole network’s mempools…