DE edition and both the article and comments are wrong.
To max EXP, make sure that you have done 3 things before confronting Lohar:
(1) Investigate Mordus' house in Driftwood and give "Letter to Mordus" to Lohar, **BEFORE** you travel to Wrecker's Cave
(2) go to Fishery factory basement and discover Soulforge weapon
(3) get Queen's Letter AND Mordus' Amulet from Wrecker's Cave and carry both with you
When you confront Lohar, the game will limit your choices so that you can only finish "Shadow over Driftwood" first, and no matter which option you choose (blame him / agree to travel to Arx) you'll get the next quest, and conversation will end.
From here, you get 3 choices to finish "The Law of the Order"
(1) You talk to Lohar again, give letter to Lohar: quest ends immediately. You get no extra exp & few garbage rewards.
(2) You talk to Lohar again, show the letter to Lohar, but don't give it to him: you fight Lohar + 3 minions + 1 prisoner under interrogation, kill them all then talk to Magister Julian. You get extra 31000 exp (including killing everyone in the room) + level 9 legendary gear as reward.
(3) You talk to Magister Julian first, then talk to Lohar again: you fight Lohar + 1 minion, then talk to Magister Julian. You get extra 25000 exp (including killing Lohar + minion) + level 9 legendary gear as reward.
Fighting Lohar will ***NOT*** make the whole undertavern hostile to you, *IF* you can control the combat inside Lohar's room. Both option (2) and (3) mentioned above are the same. After you killed Lohar and his minions, everyone else will calm down and forget what just happened. All the conversations to NPC traders will *NOT* change, as if nothing had happened at all.
(1) If you go to Magister Julian first, the prisoner will be killed and two interrogators will move to Effie's shop, and you won't be able to fight them without mass massacre.
(2) Fishery factory basement does not have a standalone quest in DE, ver 126.96.36.19948; finding Soulforge weapon will be marked as quest progress of this quest. Talking to Julian will NOT give you any option to reveal smuggled weapon, either.