A traditional cycle involves having a few bacteria accidentally drift into your bucket/tank from nowhere and then we add something that decomposes but doesn’t give usable nutrients right away, takes many many rounds of bacterial replication to get going, if you even start with any of the correct bacteria. More likely you start with suboptimal bacteria that the better bacteria then have to compete with once they find their way in. Meanwhile your decay products are building up, potentially to levels that inhibit bacterial growth, all the while just kind of hoping with fingers crossed that the biology is all sorting itself out optimally.
With Dr. Tim’s you are adding billions of the correct bacteria, ready to go to work on day 1. Then you add some fish and they pee and poop into the water giving a rich and full immediately bioavailable diet for the various bacteria, not just ammonia. Many many fewer rounds of bacterial replication are required to seed every available surface since you are starting with a lot, with the bacteria you want as opposed to whatever was floating in the air.
Fishless Dr. Tim’s with adding ammonia is also an option for those too nervous to use fish, but to me it is clear that this will be a poorer and less consistent (peaks and valleys) diet for the variety of bacteria we want to grow. Plus it leaves you with a tank full of nitrate not balanced with phosphate and so more difficult to remove later.
I don’t have citation, but I researched it a lot before I dumped my fish in a new tank without a traditional cycle, and I also have a professional background in related subjects.
I agree that if your rock has a lot of decaying matter on it you might overwhelm the added bacteria, but even then adding large amounts of the right bacteria will help tremendously. But anyway that’s why I started with prepared rock.