If you had an unlucky year and had to spend an extra $10K on various things (car breaks down, medical expenses, moving for a job) it's probably not a big deal for you. If a leanFIRE person had an unexpected extra $10K expense, they'd have to cut $10K out of the rest of their budget for the entire year.
If you're already squeezing by on $30K/year like this author, somewhere between 1/2 and 2/3 of that might go to basic living expenses (rent, utilities, food). If the other 1/3 of your budget gets wiped out by an unexpected $10K expense, it's going to be a very lean year.
This is the problem with most leanFIRE plans: They only work if nothing ever goes wrong, no unexpected expenses occur (for 4 or more decades straight), and a person's lifestyle never expands at all.
The notable exception is called a "sequence of returns risk" (SORR) where either something bad happens in the first few years draining a really large portion of your original savings (more than $10k) and/or the market undergoes a recession during the first few years, and if you withdrew the full 4% from your investments while their value was markedly depressed, you would never recover (without additional income). In my opinion, a proper retirement strategy should account for SORR; some padding (i.e. the wants portion of your budget you can reduce during a lean year), reverse-glide strategy where you can draw from cash/bonds instead of equities in case of depressed value equities, etc. In many cases, this scenario happens so early in retirement that anyone retiring at a younger age has relatively good prospects of rejoining the work force to get to the other side, and then will likely be very well prepared for a second retirement with a decreased likelihood of yet another bad sequence of returns occurring before their nest egg has grown well beyond 25 times annual expenses.
And all retirement plans should be flexible - some years where you might spend a bit less than the target, but have room to change that, particularly if your invested assets grow beyond the original necessary funds.
I don't think that's the case if you're retiring early. The 4% withdrawal rate was based on a 30 year retirement. You need to go a bit lower if you want to have minimal risk of running out of money for a much longer horizon.
You're more likely to die than run out of money!
That's a much higher level of risk than I would accept.