[dervjd]

"It’s always DNS" is basically tongue-in-cheek expression, because DNS issues are so frequently the cause of weird outages.

Almost anything you do on the internet (or local network) depends on DNS functioning correctly. DNS can get complex quickly - multiple servers (caching/authoritative/recursive) and protocols = lots of opportunities for something to be misconfigured. Cached entries in particular can be a nightmare if something gets outdated - it takes time for an update to a DNS record to propagate to all the other DNS servers on the Internet. All kinds of other random services etc depend on DNS records being correct and DNS working. When there’s an issue it’s not always immediately apparent that a DNS problem is the root cause, leading to lots of time chasing your tail/tearing your hair out trying to figure out what the heck broke.