You're correct that "standout" doesn't match the terminology in
ECMA-48.  The use of "standout" was chosen to follow the terminology
in prompt strings, I believe.

This matched the behavior of most terminals at the time (notice
${(k)color[(r)standout]} is actually from Sven all the way back in
2001).  Also, there's no way to directly reference an escape sequence
for any of the "Attribute Codes" (they aren't in the $fg / $bg
hashes), so those values are mostly there for commentary value.

I would have no objection to adding "italic"/"no-italic" to the color
hash.  It also appears standout would more accurately be tied to
reverse-video now, but because the keys of the hash are the numeric
codes we can only have one or the other.