[flat.multimap.overview] p4 says:

Except as otherwise noted, operations on `flat_multimap` are equivalent to those of `flat_map`, except that `flat_multimap` operations do not remove or replace elements with equal keys.

[Example 1: `flat_multimap` constructors and emplace do not erase non-unique elements after sorting them. — end example]

This doesn't really work, because there is no `flat_map` operation that is equivalent to the constructors and `insert` and `insert_range` functions that take a `sorted_equivalent_t` tag argument. The `flat_map` operations that the wording intends to refer to take a `sorted_unique_t` tag argument. It's a bit of a stretch to say they're equivalent when they don't even have the same parameters, so we need some weasel words here.