Being in 'this realm' and a developer for many years, I assume they did not stress test at all, due to their knee-jerk reaction to having to spin up a new instance.
I did check the numbers, and peak yesterday was 55K. Smoke testing, and load testing, could have -easily- proven out 5 digits worth of people, and emulated their login, spiked delays, and done a lot of other tasks to avoid this.
Majority of these bugs could have been sated, and grown even more hype, by doing a couple phases of a CBT/Stress Test.
THAT being said, they are a small, new, company so I am not surprised they didn't know what/how to test. Where they are fucking up is the lack of communication, and lack of day one patches. The Suggestions/Bug boards are littered with people complaining, and I've not seen any replies -at all-.
With Steams return policy a lot of people will just return it, and maybe come back later. And no, this isn't a "good thing" - That is lost recommendations, and won't get a 100% return on the people who leave.