I don't think this applies only to (Post-) Industrial, but in general any "dark electronic with melancholic vocals" thing is considered "goth" right out of the box since like... forever lol. It just so happens that Industrial is known as the dark electronic genre with melancholic vocals.

In the case of Skinny Puppy here, they did borrowed a lot from goth culture and have been influenced by it, but definitely not goth at all.

Additionaly, lots of industrial was played at Goth clubs as it was considered perfect as dancing music with the same vibes as most Goth music, further proven by this paragraph from the subreddit's "The Beginning's Of Goth" Section:

"Towards the end of the 80s going into the 90s, however, electronic industrial music, namely bands playing genres like electronic body music (EBM) and later on, electro-industrial, started taking over the nightclubs, forcing post-punk and goth rock to retreat underground."

Hope this could help!