Why didn't they though? Why did they become a monster that, as we are seeing right now, can survive easily with about half of its employees gone in a heartbeat?
A bit premature to make this claim imo
Plus, you don't grow to a tech behemoth (Google, Microsoft, Facebook, et c.) by running smart & lean. You might get bought by them that way (cf. WhatsApp, and maybe even Minecraft) but you don't become one of them. So if leadership/investors thought Twitter had a shot at joining the big-boy club, maybe that's why.
Great companies focus on business performance not headcount. I'm more impressed with leaders that generate more positive metrics per dollar. Sometimes it takes headcount, but most of the time, a really great exec can make more revenue/growth/market share happen with the same or less headcount.
In the US, only about half of college graduates have jobs that require a college education.
Craigslist has about 50 employees. If you really had to, you could probably run Twitter with a very small staff, and the computing outsourced to AWS.