As Rancid Crabtree suggests, I suspect your bare aluminum threads have seized due to several combined corrosive effects, and that the soft aluminum could be easily damaged by wrench distortion and high torque.
There may also be rough machining or debris from manufacture.
I see no cap O-ring seal on the drawing posted, (but a gasket or O-ring could be installed on the cap assembly).
As fps plus suggests, using a cloths iron or similar to heat only the cap with direct contact can expand the cap with less heating (and expansion) of the housing.
Speed and timing are critical as you need to catch the cap expanded before it conducts heat to the housing.
Multiple heating and full cooling cycles and tapping with a nylon hammer may be needed.
If penetrating oil (I use ER gun oil, Dexron/kerosene 50/50) is applied to the cap & housing junction after heating, it may be drawn into the threads as they cool, and subsequent heating & cooling cycles (and time) may also help to further distribute the lubricant.
Be patient, several heating cycles may be required, do not get in a hurry and apply destructive force.
I suspect when it finally releases it will be little more than finger tight.
Thread lubrication and anti-seize compounds are a thread’s best friend.
‘I use that stuff on everything.’
http://www.neverseezproducts.com/antiseize.htm
http://bostik-industrial.com/never-s...ing-compounds/
“Resists galvanic action”
“Prevents galling (friction welding)”
“Protects against corrosion”
I prefer any of the grey or copper anti-seize compounds, but IMHE any grease is better than dry threads.
Good Luck, YMMV.