Well that doc has (4.3.3), which only applies to incomplete dates (eg: no seconds):

the expression shall either be completely in basic format, in which case the minimum number of separators necessary for the required expression is used, or completely in extended format, in which case additional separators shall be used in accordance with 4.1 and 4.2.

4.4.4.1 also has:

provided that the resulting expression is either consistently in basic format or consistently in extended format

but that's for an interval, so it's mostly about having the format for the start and end (although the chosen phrasing also means consistent format within the dates). Same in 4.4.4.3 for start + duration and 4.4.4.4 for duration + end. And there's 4.4.5 which is the same as 4.3.3 for duration rather than single dates.