Obtaining Admin access on Twitter probably can't take down Twitter. Although, an admin might be able to run heavy tasks that a normal user can't access to DOS the service. That hypothetical would be even easier for sysadmin's to track down though since I believe only a single admin account was compromised.

Not really. The Twitter hack was a community admin (account data, customer service rep), not a sysadmin (software config)

Twitter isn't anywhere in the same league as Apple/Google/Microsoft in terms of security.

It was an anecdote. If a 17 year old can socially engineer twitter, I don't think it's crazy to think that a room of comp sci PhDs in South Korea couldn't figure out how to mess with infrastructure, in fact I'm quite sure they could. There are not 3 countries in the world who could cause this type of outage, there are many.

Do we know a lot about Apple's internal reliability/security engineering? Google writes books about it and has things like Project Zero. Windows is used by governments everywhere and so Microsoft probably has some contacts in high places. I have never heard anything about Apple.