[culi]

This is a typical API design in Ruby, but the post is about a somewhat novel API design than what you're pointing out. To address some of your points:

The exclamation mark is a convention. It is used whenever a method could possibly result in an exception being raised. Sometimes it's instead used for non-idempotent methods.

"3.days", etc are Rails things. A lot of non-Rubyists don't like it but once you use it for long enough you tend to really grow to it.

As for HashWithIndifferentAccess, yes this is generally acknowledged as a mistake in Ruby's design and is rarely used in my experience. Originally, all Ruby hashes were HWIA. When they finally realized this was a design mistake they had to create HWIA for some level of backwards compatibility