You don't understand that the choice of underground vs open pit...

You don't understand that the choice of underground vs open pit is an economical decission based on the geometry and depth of the ore deposit. If LTR have decided to go underground at Kathleen Valley is because the more interesting areas of the deposit are quite at depth and reaching them digging a pit from the surface would have had higher costs (incredibly more tones of waste material to remove and stock elsewhere) than going through portals and ramps.
You don't understand that lithium is never mined. What is mined is an ore (an association of minerals). To mine/extract an ore from the ground and take it to a metallurgical plant is more about the physical features of the material than the chemical ones. That is the reason of hiring mining engineers and not chemists for that purpose. Lithium is a chemical element that is never found free at nature (as opposite to gold or platinum-group elements). Lithium is enclosed in lithium-rich minerals (spodumene, lepidolite, petalite...) that are specially abundant in pegmatite. Pegmatite is a hard, brittle rock that is easy to break up. Far easier than sulfide ores (nickel, copper) that are less brittle (more ductile) and tend to stick to metal components of mining tools.
You don't understand that current spodumene mines in Australia did not gone underground from the beginning because their deposits are shallower than Kathleen Valley and is was cheaper to reach the pegmatite in open cuts. However there is more waste material to remove. Greenbushes has had underground operations in the past to extract spodumene. If they are no longer active is for economical reasons (ore grade and extraction cost) and not for technical issues.
https://portergeo.com.au/database/mineinfo.asp?mineid=mn508

Each ore deposit requires its own solution to extract the ore, and every mine will choose the solution that maximizes economical benefits. So has done LTR at Kathleen Valley. Underground mining has been done for millenia worldwide (not in Australia, of course). Pegmatite extraction from host rocks is technically feasible. LTR has performed detailed geotechnical drilling and studies and has concluded that the ground features are excellent for underground mining.
https://company-announcements.copyright link/asx/ltr/36644b67-980d-11eb-8dae-96f1aa3153e8.pdf