Engor, as far as i can tell Cabin Lake is actually in Banff National Park. The park/provincial border is indeed, as you describe, west and south of the lake, putting the lake within Alberta. The Canadian Rockies Trail Guide seems to confirm this:

excerpt from 75 - Owl Lake - Marvel Pass

'A small lake on the pass provides a pleasant rest stop and viewpoint and is also a bit of a curiosity. Though its waters drain west into British Columbia, it was mapped as part of Alberta and actually lies within Banff National Park. A partially constructed log cabin sits on the lake's north shore, where a British Columbia resident started building in the 1970s believing he was on the BC side of the border; he was informed otherwise by the park authorities.'

I doubt anyone would give you much grief for random camping so near the park boundary but as the lake appears to be inside the park it isn't technically legal.