Rubber band drive….

[GSXKING]

I'm certain my HD never had a tensioner.
It could be adjusted like a chain though.

[Eric GSX1400K3]

I dare say there would be enough stretch in the belt to provide for the slight geometry change from the swingarm pivoting, similar to the slack in a chain drive.  I do agree that a tensioner makes sense, can't see one in the photos provided, perhaps behind the clutch slave cover?  More critical in timing belts than drive belts though.

[Hooli]

It seems to me that there should NOT be any slack on belts, at any time, should they be cam belts or transmission belts.
I don't see any belt tensioner castor/pulley here, as opposed to what can be easilly seen on Buell bikes, for instance.
If there is not, how does the guy cope with the swingarm... swinging and the geometry change that normal chain slack is there for?

There is slack, just less than a chain. From memory on my HD it's about 1/4"-1/2" Too tight kills the bearings, just like an overtightened chain. Possibly why it's tends to be a popular design on cruisers that tend to have less suspension movement than some other types of bikes.

[Cikik]

Most bikes normally fitted with belts have more shrouds to prevent stones/chippings interfering with the belt/sprockets.
Stones/chippings can make short work of a belt.