Cave Spiders still spawns in the vicinity of Magnum Torch (https://www.curseforge.com/minecraft/mc-mods/magnum-torch-forge). No spawner near.

Yes, Magnum Torch only prevents natural mob spawning, according to the mod's curseforge page.

Is magnum torch intended to stop structure mob spawns? Mobs that spawn as designated by a structure's json work differently than normal environmental mob spawns. I wouldn't be surprised if magnum torch is intended to only stop natural environmental spawns, and not structural ones.