Ugh, skill swap chains.
Anyhow, just tested it with normalize skitty relayed to skill swap reuniclus relayed to a volcarona with will-o-wisp, against a wild drifblim in double grass. It burned drifblim, yes. And to be sure I did it right, I used bug buzz from the same volcarona on drifblim the turn after, where it gave the "It didn't affect Drifblim" message.
So yeah, will-o-wisp is akin to, say, hypnosis on dark types, where the typing of the move in itself makes no difference on what it can and cannot affect.
Anyhow, just tested it with normalize skitty relayed to skill swap reuniclus relayed to a volcarona with will-o-wisp, against a wild drifblim in double grass. It burned drifblim, yes. And to be sure I did it right, I used bug buzz from the same volcarona on drifblim the turn after, where it gave the "It didn't affect Drifblim" message.
So yeah, will-o-wisp is akin to, say, hypnosis on dark types, where the typing of the move in itself makes no difference on what it can and cannot affect.