You're right, Slipknot aren't a goth band, they're a nu-metal band. Back in the early 00s there was confusion in the media (and therefore, most people) between goth - as you said which encompasses Post-punk, Goth Rock & Darkwave, etc and what was dubbed
'Mall-goth' in America - this is basically what we in the UK called Greebos/Moshers, (and later, Emos) but also got shoved under the 'goth' umbrella due to the American name and visual similarity in aesthetics. The Wikipedia page I just linked defines it as "Goth for fashion rather than culture". Generally Mall-Goths/Greebos/Moshers listened to Slipknot, System of a Down, Limp Bizkit, Korn, Rammstein, Marilyn Manson, Deftones, Kittie, any and all nu-metal and also grunge bands like Nirvana, Soundgarden and Pearl Jam.
Here's a couple of
articles on Greebos/Moshers in the UK if you're interested. Also, crucially, many were too young to attend club nights where the older goths like you were talking about would be going on a saturday night,
but could get into gigs and festivals as long as they looked 16.
Generally Greebos/Moshers/Mall-Goths wore baggy jeans, band t shirts, had grown out hairstyles and often dyed their hair black. Mostly they looked similar to Goths but with less back combing and crimping and more hair straightening. Inspiration being less Siouxsie and the Banshees and more Murderdolls. Funnily enough I would likely be called a gatekeeper for this lecture by a lot of Mall Goths though, because since there's societal stigma about 'goths', that increases the desire to be labelled as one.
(I used to be a Greebo/Mosher/Mall-Goth and later moved into the Goth subculture because I discovered I love Joy Division, the Cure, Sisters of Mercy, etc when I got to university and got to go to actual goth nights and events. When I was young I didn't accept any label except 'alternative' though)