I'm not sure that listing known locations is available, so we'll need to be creative on this one.
You could get a list of existing calendar events and only select the locations property (what you linked to). If you then filter these client side to remove duplicates, you can get a decent list of known locations. The down side is that if a room is now available but has never been used, it won't appear in this list. Also, this is only for one user so another idea is querying a different (or multiple) calendars. For me, the following snippet returns around 247 unique locations.
https://graph.microsoft.com/v1.0/me/events?$select=location&$top=500
With the Graph JavaScript SDK this looks like:
client
.api('/me/events')
.select('location')
.top(500)
.get((err, res) => {
var locations = res.value
.map((x) => x.location.displayName) //only get the displayName
.filter((v, i, a) => a.indexOf(v) === i) // remove duplicates
});
There's also some existing threads about listing conference rooms.