Not all of it. You obviously missed the link in that article to his FAQ answering every objection you raised: http://earlyretirementextreme.com/frequently-asked-questions ...a max of $1200/yr on food
which means he has to work on that
Right. His point is that you don't need income, not that you don't need to work.
and he conveniently didn't calculate the opportunity costs
Zero if you like gardening.
tools
For gardening? Amortized over the life of the tool? Visit some estate sales and you can get set up for gardening like a pro for probably $50.
irrigation
You mean like a rainwater barrel + some old rubber hosing? You can make one of those for probably $10 in parts.
chemicals
Not necessary.
seeds
These come from the plants themselves. As in, free.
What about transportation costs
If you don't commute and ride an old bicycle (know how many of these people give away? and can be fixed for <$20?) these can be surprisingly close to zero.
Anyway this is in the FAQ he linked to. $600/yr
electric/gas, phone
This is the first thing in the FAQ he linked to in that article: $3240 per year
internet
$240/yr he said
household supplies
Well, you found one thing he missed.
upgrade of your old computer, clothing, shoes
Did you miss where he said he has a 7-yr-old computer and keeps clothes for a long time (= very low amortized cost; probably $50/yr total for these things)
Then if you want to have a car
He covered this. $600/yr
don't pay property taxes,
Read the FAQ. He estimates a you could afford a $100k home on that budget. I've lived in one in an expensive state (RI), it was spacious.
no safety net in case of a serious sickness.
That's what savings are for. $3000 in savings and bam, you've covered your deductible.
TLDR: read the article and the FAQ.