So if the next ECMA spec adds methods like map and filter etc to everything that is an iterator, does that mean Set and Map also get them?

Yup Set and Map have methods that return iterators (including .keys and .values and .entries)

But with this proposal, we could simply accept an Iterable or IterableIterator and a consumer could pass arrays, Set, Map?

You can do that stuff today, but having the helper functions on the Iterator class itself is a huge added convenience.

Today you could write something like this:

function* iteratorFilter<T>(it: Iterable<T>, condition) {
    for (const item of it)
        if (condition)
            yield item;
}

const map = new Map(...);
const matchingValues: Iterable<T> = iteratorFilter(map.values(), matcherFn);

With the builtin helpers the same code would be:

const map = new Map(...);
const matchingValues: Iterable<T> = map.values().filter(matcherFn);