Web Server: IIS, Database: SQL Server, Web framework (backend): ASP.NET/MVC. Additionally: Redis, Elasticsearch, HAProxy (all on Linux). jQuery on the front end (with some TypeScript) and LESS -> CSS.
Automation costs are actually administrative costs (platform fees), which have recently increased via unmonetized opportunity cost models (opportunity being a specific technical term in advertising). SO is not fully or optimally monetizing, probably because they prefer to keep the old tech they are using and/or the staff that has been there since early days. It's also possible they want some dejour/nonstandard controls. Saying they are making an informed tradeoff is simply incorrect. Saying they are making a (unspecified) choice, then not really talking about that choice, is what the article skirts.
Sub 10k/mo for platforms at 2 billion opps per day is AOL and SpotX. That schedule doesn't matter at the volume SO works with. Fees and Opp costs are not the issue with the unmonetized inventory.
Wether they choose to move to linux is a different matter.