Let's ignore Hornet/Thief Bug since we don't have numerical results, just anecdotal evidence.
If target hard DEF gets converted to soft DEF specifically for DB, DEF reducing damage directly might make sense, If Senshirou is the one that cast Stonehard Skin on the other player, the DEF boost from Stonehard Skin would be 140 (56 * 10 / 4). The Stonehard Skin version took 141 less damage than the normal version.
Provoke is harder to calculate because we don't know what level of Provoke was used, but since I'm assuming the target DEF was 295 before Stonehard Skin and Provoke...
Normal DEF (soft + hard): 295
w/ Stonehard Skin: 435
after Provoke: 402
Provoke only affects hard DEF, but unfortunately we don't know what the soft or hard DEF of the target is, and 295 total DEF is just an assumption.