Near its start, one readable said "30/03//386". I also read the name "Truart". I immediately thought: ah okay, so this takes place shortly before Thief 2 itself.

You can throw all that connecting out of the window straight away.

This mission defies logic, especially in the storytelling department. And that's what makes it shine. It's like one of these old LEGO Star Wars bonus levels. Things you see probably aren't real. This level plays like an extreme acid trip. Sometimes the trip goes good, sometimes it goes bad.

The atmosphere always carries an uneasy undertone. Occasionally, there's definite scares to be had. At other times, things are almost humorously weird.

The gameplay isn't always as great. Things can feel clunky. Near-unavoidable red-hot pipes in a small room to cross, navigation that makes little sense, and a very frustrating beginning all play a part in this. Still, if you can stomach some horror elements, and don't mind any sense of... sense being thrown out of the window, you will find an art piece of a mission.

The plot (or a system of fleetingly connecting sections of different plots in a day in the life of the City that Garrett is fleetingly witnessing) makes perfect sense. Read the readables/hear the conversations/observe the details. Do not bother connecting everything to the basegame timeline or making a sacred cow of it. Thief at its core is best as dark fantasy which many authors nowadays trying to be faithful chroniclers of a non-existent world instead of creating their own enrichments of it (like this mission does) forget.

I did the exact opposite? I praised the mission for not adhering to connecting timelines or anything. It feels kind of belittling to talk like this. I accept the disconnect to proper worldbuilding - explicitly stating so - and someone decides to tell me that I should accept the disconnect to proper worldbuilding. I don't really need that. People should be able to engage in whatever way they want to anyway. If someone dislikes the strange and more subtle, off-kilter way of storytelling, that's completely fine. It's just a matter of what people want to engage with when they play Thief.

And clearly there's little sense to the mission's worldbuilding. Mechanists just popping up near the beginning made absolutely no sense (and I love it for that). A guard smoking weed while on duty is hilarious, and nonsensical.

Well if weed is your 1st association ;-o I always interpreted it as just smoking to kill time (cool enough as is). Mechanists are there apparently on notice to collect bodies for their certain project...etc.

PS You made it sound as if the story wouldn't fit into some timeline but it wouldn't matter anyway because it would be all nonsensical - it is not. It is relatively difficult to parse the story from all the relevant readables that need to be combined with observations from environmental storytelling (I hear even native speakers have problems with all that) but not to the point of obscure - it is more just about paying attention to detail that is laid out to you. Just some small examples (SPOILERS): one readable in the underground sancturary describes the procedure for invoking the monster/demigod ("idiot god") using among other ingredients freshly baked bread - there's a baker's oven still firing not far away in the same place. A readable in the Balnebrich mansion implies hiring (or more precisely luring) a professor - expert lexicographer (obviously to decipher the tablets) who is nowhere to be found in the palace or anywhere else - until we learn his fate in a suggestive encounter in a far darker place. Of course there's also sprinklings of surreal but such should be any good cinema, and magic has always been inherent to the OG world anyway.