The undertone was intended to be: that's an insane amount of money, something one with quadruple that amount of experience would maybe earn in a for-profit organisation, but I guess your reaction further proves it's different where you're from
It's not high for bay area software jobs; there are new grads who were paid more than that 10 years ago and I assume new grad wages have gone up since. Of course cost of living (particularly rent) and taxes are high there too, but if you don't blow it all on renting a higher-end place or luxuries you can still save a lot.
Does this mean you get benefits (like free housing, healthcare, and money to buy food with) if you earn less than 105k/year? Or what does low income threshold mean here
It's one criteria for eligibility for social benefits that can include being able to live in certain kinds of public housing. Usually there's a lot more criteria that go into it, but income is a fairly major one.
