You are right that stout is a modern (read: hopped) style, but it grew out of older English traditions in which hops was seldom used. The very name "Mugwort" implies that it is an herb (a "wort") used in the "mug" (for ale).
I don't think it is inaccurate to call that Mugwort "stout" a modern version of a medieval English strong ale or old ale. The addition of honey and molasses to increase aging potential, as well as mugwort and licorice are very historically accurate. Many old ales were also made without the use of any herbs whatsoever, hops or otherwise. And because of the more primitive techniques for roasting malt, it was common for some of the malt to be charred due to uneven roasting, giving the beer a stout-like quality.
If we wanted to make it even more historically authentic, we'd use oat malt in addition to barley malt, but to my knowledge oat malt is not commercially made today so we have to settle for flaked oats.