You have to always decode.
No, you don't have to with US-ASCII and iso-8859-1.
But that wasn't part of Juerds claim.
I agree. He didn't mention any relation between the first 128 characters of iso-8859-1 and UTF-8. No idea why you bring this up.
iso-8859-1 is an encoding (of a subset of Unicode)
Unicode is a character set, not an encoding
, so that sentence is broken.
iso-8859-1 is both a character set and an encoding. The iso-8859-1 character set is a subset of the Unicode character character set, but this property does NOT explain why iso-8859-1 works without being decoded.