Does anyone know if the maps in the array returned by ResultSet.asMappedResults() are ES6 style maps? The docs say "Returns the query result set as an array of mapped results. A mapped result is a JavaScript object with key-value pairs."

Not really, there's no

size

property, for example. A number of other Map functions are also missing, although each result does have a forEach function available, which will loop through the properties of the result (the columns you include in your query)

Sounds like an ES5 map (JS object) then