I was making 90k in the Boston area within the past few years.
On that salary I was able to support two adults, pay back student loans faster than needed, max out my 401k (company match up to $10k or something like that) and build saving at a rate of about $1k/mo, go out to dinner every 3wk or so and have enough to direct meaningful amounts into my hobbies and other discretionary spending.
That said, if you try to live like you're wealthy (shopping at whole foods, $2k+ rent, going out to drink, take uber everywhere, expensive cable, phone and internet, etc, etc) and generally hemorrhage money I can see how one could live little better than paycheck to paycheck on 90k in the Boston area.
Metro Boston/Cambridge is pretty expensive these days as are some especially tony suburbs but you can get out toward reasonable property/rents relatively quickly. And, with the exception of newer outposts of companies from SV and elsewhere, a lot of existing tech is outside the city anyway.
https://en.wikipedia.org/wiki/List_of_California_locations_b...
I'm not saying 90K is a lot, but it is complete hyperbole to say it isn't sufficient to live in the Bay. 90K is right up there with the highest median household incomes.