If you look at some startups, it seems like they're simultaneously frugal and not frugal. E.g. early Google 1999 paid for meals cooked by a chef and onsite massage therapists even though they had no revenue and profit until 2002 -- but on the other hand -- they were very frugal with spending the least amount of money for computer parts[1] and datacenter rack leases.
Likewise, Amazon was famous for being cheap by having employees make desks out of doors but they were bold in spending money on acquisitions that were strategic to their goals or taking expensive risks on Prime's "free shipping" getting abused by customers.
The way to reconcile the inconsistency is that being frugal can't be applied in every area of the company.
So maybe a more realistic tactic for a new YC company is ... it's ok to splurge $5000 on an automatic cappuccino machine in the office for employee well-being, but at the same time, be ultra frugal in AWS costs and analyze the line items like a hawk to make sure the developers are not leaving up idle EC2 instances and wasting money.
[1] https://commons.wikimedia.org/wiki/File:Google%E2%80%99s_Fir...