Carbureted Rotax engines have no mixture control, how do they handle less dense air?
According to this owners manual

4.5 High altitude compensator (H.A.C. kit)
H.A.C. is a high altitude compensator developed by ROTAX which adjusts the air/  fuel
mixture automatically from sea level to 6500 m altitude using a
special BING  carburetor.

This page has information on how the Bing carburetor works (with pictures!):

Here's how it works
Did you ever wonder what makes fuel flow up the jets, from the
carburetor float bowl, enroute to the engine? Is it vacuum at the
venturi?  That's not quite descriptive enough. It is better defined as
a differential pressure between the float bowl and the venturi. What's
the pressure in the float bowl? Here is an important hint, the bowl is
vented to outside atmosphere. So, it should be the same atmosphere, or
ambient pressure, that's feeding the airfilter.  If it is not,  the
differential between the venturi and float bowl is going to change and
so will the mixture...Maybe richer, maybe leaner. Just think of that
for a second. If  fuel air mixture can be thrown off, accidentally by
improperly venting the the float bowls, why not control the venting to
effectively control mixture? That's exactly what your Bing 54 can do,
automatically, when a High Altitude Compensator is added.  If
pressure in the float bowl is reduced, relative to the venturi,  less
fuel makes its way up the jets, and consequently, mixture is leaner.
The HAC unit has two chambers separated by a diaphragm. One chamber is
completely sealed (except during calibration) and air density within
remains constant. The other has airfilter, float bowl, and venturi
connections, so air density on this side is variable. It is in this
chamber, that ambient pressure can be reduced by feeding it to the
venturi via a connection on late model carburetors. The amount of
ambient reduction is controlled by a tapered needle which changes
position with deflection of the diaphragm. Think of the diaphragm as a
flexible wall between two chambers that allows the sealed side to
expand or contract as changes in ambient pressure occur on the other
side. The reduced  (from ambient) pressure gets routed to the float
bowl(s) via their vent lines with a resulting leaning affect..  If
that supply of low pressure is shut off, float bowl pressure returns
to ambient, and the mixture goes as rich as the jets will allow. With
HAC installations, standard jetting starts out several steps richer.
This is why we say the default, or failure mode is typically toward
the rich side.