Can not vs must not
I think what they are trying to say is that you can't use cabinet fronts to cover the front.
The manual on their website (if we are talking the same model - UOFZ124) shows the condenser to be completely in the bottom of the machine.
And it only occupies the left side so air can completely circulate right side in left side out.
I would dare to say it uses a fan for air flow since with that horizontal installation, convection alone probably won't do much.
So, if the handle on the front fits behind the cabinet door, there should be enough space for air to circulate around the ventilation grill at the front bottom.
How and where you ventilate the cabinet itself shouldn't matter then as long as enough ventilation is there in general.
So if you put a cutout in the door or the top of the cabinet shouldn't matter.
The more air flow you can provide the more efficient it will run.
The more efficient it runs the longer it will live.
But it should run perfectly fine with just a few square inches of ventilation area.
And I think that at least a certain amount of that cabinet thing is about it being R600a and if it leaks, an enclosed space could lead to an explosive atmosphere building up.
