It being a free country and all that, one can calculate a savings rate however one wants - there is no law specifying a particular way.
If you want to estimate "how long until I can...?" or "can I...?" retire, you can ignore the mortgage payment (principal and interest) altogether.
Let
E = non-mortgage living expenses in retirement
WR = planned withdrawal rate as in the Trinity study
RMP = Remaining Mortgage Principal
S = "the number" needed for FI, aka the stash.
Then one can calculate/estimate: S = E/WR + RMP
Only assets that will be drawn upon to pay expenses, and payments directed into those assets, are counted to calculate/estimate current and future values of "S". The monthly mortgage payment is, as noted above, irrelevant.
Of course there are other ways to do this, and the approach above assumes the mortgage is being used to pay for a home, not a flippable house. Given that, it is as defensible as other simple methods.
Note that several of the above posts say pretty much this, albeit in slightly different words.