I think folks with clearly-identified dates fall into a couple of categories:
1. People with triggering life events (this is us, a/k/a "DS goes off to college," because then we will be free to do the FIRE travel we want)
2. People who are largely relying on definitive income streams (pension/SS) and so aren't as subject to the vagaries of the 4% rule
3. People who really hate their jobs
4. People who are "talking" with apparent certainty in the context of the forums but who are aware of a ton of unstated caveats (also us, btw)
5. People who are very close to FIRE and so have a much clearer sense of their needs and the available assets.
6. People who are blowing smoke and assuming a certainty that doesn't exist.
FWIW, one thing we have found helpful is making things feel more certain is the "buckets" approach that I first read here (I think from Goldilocks): you look at your retirement as a series of time periods, each of which will have different sources of income and different levels of expenses. You start with the farthest-out (for us, 85-100): I estimate annual costs of X (e.g., high medical, low travel) and annual income of Y (e.g., one SS + half DH's pension), so that leaves an annual delta of $Z x 15 years. So in 35 years, I need to have $A left to cover that. Take $A, discount to present value using conservative growth assumptions, and that is how much I need to have invested right now to cover my 85-100 bucket. Then repeat for other periods based on changes in needs and income streams, working backwards to the present. When your "needed 'stache" for the current period matches your existing 'stache, presto, you a FIRE'd.
For us, this "feels" like a far more accurate estimate. We assume our first decade post-jobs is going to be the most expensive, because we plan on a ton of travel, but we also assume that will drop off as we get older/more infirm/see the places we have been dying to and get tired of so much motion. At the same time, that first decade is going to have the lowest alternate income streams, because it's pre-SS and pre-pension. So if you looked at the 4% rule, it would tell us we need a stupidly massive pot of money as of our FIRE date, because it would assume we need to cover that first year's costs forever.
DH has done a variety of versions of the bucket assessment, showing the effects of different FIRE dates and different post-inflation returns, to tell us how much we need to save each year to fill all those buckets under each scenario.* So that range of options gives me a pretty high degree of confidence that we can meet our 2025 target date, barring some horrible event -- and it is patently clear that even the worst-reasonable-case outcome means only another year or two of work.
*FWIW, DH's assessment of our future budget needs is patently ridiculous in and of itself, which provides another massive margin of error. I would be much more uptight if I didn't believe we were already FI and all the rest of this is gravy.