It seems this plugin makes the assumption that L is still exported globally.
However using leaflet the ES6 way with direct imports like
import { map } from "leaflet/src/map/Map"

does not expose a L global anymore. So trying to include this plugins MarkerClusterGroup fails cause of this definition.

export var MarkerClusterGroup = L.MarkerClusterGroup = L.FeatureGroup.extend({

The correct way would be to write this like this:

import { FeatureGroup } from "leaflet/src/layer/FeatureGroup"
export var MarkerClusterGroup = FeatureGroup.extend({