Are Window's SIDs in AD case-sensitive?

From the same link you've provided in your question:

The formal string syntax is given in section 2.4.2.1.

Alright, let's follow that link and read that page (emphasis, mine):

The SID string format syntax, a format commonly used for a string representation of the SID type (as specified in section 2.4.2), is described by the following ABNF syntax, as specified in [RFC5234].

Sounds good, click another link... some more reading...

NOTE:

 ABNF strings are case insensitive and the character set for these
  strings is US-ASCII.

A Windows SID structure always begins with S-1 - an uppercase S, a dash, and a 1 - denoting the data type and revision (always 1).

So yes, the SID structure is case-sensitive; BUT SID consumers, like Windows itself for example, does not treat SID's with case-sensitivity

A SID is a variable length binary structure. The "standardized notation" for representing it as a string does seem to have a capital S but it really depends on what is parsing the string and whether it will care if the S is capital or not.

That ANBF format isn't 100% accurate: it specifies that sub-authorities can be represented as a 10 digit integer, but 99999 99999 (max value for 10 digits) is outside the range of a ULONG, which is a 32-bit integer. The actual maximum value is 2^32=4294967296.