Can someone explain why exactly this can be cast into combat?

Unless stated otherwise, the following rules apply to all spells:

Forward arc, does not need line of sight, lie within the spell's range, cannot target spells at units engaged in combat.

The spell description says you can affect a single infantry unit with 24" and line of sight of the caster. It does not give you special permission to ignore the other requirements (into combat).

Here are some skaven spells that *do* give permission, explicitly- cloud of corruption, plague, wither.