It is not generic advice.
But in the USA especially many of us feel that long term interest rates are artificially low, certainly last year when they were 3.5%, but even now at 4.5%, fixed at nominal rates for 30 years.
Now, no matter what your personal investment portfolio is, whatever your allocation between stock, bonds, real estate, tulips, .. if you are planning to FIRE there will be an assumption on SWR. MMM recommends 4%, not out of line with generally accepted rules of thumb and historic performances of investments.
But anything similar, even 3% SWR, inherently projects long term average CAGR nominal growth in your portfolio of about 7 - 9 %
so borrowing long term at these low rates as long as you can handle the cashflow implications and volatile portfolio ups and downs, tax implications too, seems a solid investment.
This must be considered in line with your overall portfolio gearing ratio - which should be low, and preferably only comprise this kind of long term fixed deal.
In other countries, interest rates are much higher, and usually floating. Fixed rates can be more expensive and only available for a few years. In these cases especially paying off that mortgage sounds like a great idea, because relative to your portfolio the risked returns are better, or at least comparable, in paying off the mortgage.
Note though that putting all your money and savings into 1 asset, your house, is also pretty crazy and very high risk.