To me it sounds like they came up with them in 5 minutes and threw them into the studio and said "done".
That's applicable to early Mastodon but definitely not CTS. You just bein' straight up retarded, son. What kind of riffs do you even consider creative (from any band)? The ones from CTS were some of the only riffs of any band's album in recent memory that I actually remembered and liked.
"Creative" can be anything really, just something that sounds fresh, full of life, charisma and energy to me. That's what I look for most in music and I tend to think that's what my tastes reflect.
If you want examples....
Candlemass - Solitude:
https://www.youtube.com/watch?v=6Spu7FWs8ikWait for
DAT RIFF at :41 to kick in. I love the ever loving fuck out of that riff and its definitely one of my favorites ever. So much color, so much energy, so much life in that one simple riff, none of which I got from anything from Crack the Skye.
The Lord Weird Slough Feg - The Spinward Marches/High Passage/Low Passage
https://www.youtube.com/watch?v=LORTX3Yot2kThis is technically two songs but I listen to it as a single piece all the time. The main riff of The Spinward Marches has just an enormous "in your face" kind of charisma to it.
At High Passage/Low Passage, the thumping bassline adding to the main riff just sends me into musical oblivion. There's something so insanely catchy about it, and the production lets the energy of the song really shine (And the fact that Mike Scalzi's vocals are absolutely godlike doesn't hurt anything either).
I have other examples if you want.