It's not a model so much as a long look backward, based on actual history and a set of assumptions. The classic 4% SWR itself isn't a guarantee of any kind, it's based on a 95% probability over a 30 year period. Even if you change your assumptions to reach a 100% result in FIRECALC, that only tells you your assumptions would never have failed for the 30 year (or whatever you choose) periods from 1871 to present. That is still no guarantee of what the future may hold, if the future somehow differs from the last 140+ years. That's for each of us to grapple with.
And it was never meant to be 'set it and forget it.' Blindly following the withdrawal methodology for 30 years without ever re-evaluating would be foolhardy, and NOT what any credible source has ever suggested. It might work, it might not. The prudent approach is to be more conservative withdrawing (at least at the outset) and/or re-running FIRECALC every 5 years (or whatever frequency you choose) and adjust spending or income accordingly, up or down.
FIRECALC is a framework for spending your retirement portfolio, an axe not a scapel, rough order of magnitude. It is not a hard and fast spending plan by any means, there is no such bulletproof plan no matter what the cost or source...